[MB-13510] Add gci linter to make go import order/grouping more consistent

[reggieriser]

Jira ticket for this change

Summary

I've noticed that the order of our go imports has seemed a little random and getting more so over time. Ideally, the imports would be organized as follows (alphabetically within a section):

import (
    [system packages]

    [third-party packages]

    [project packages]
)

However, when a blank line is added between imports, goimports won't regroup the grouped imports to be with other similar groups. In this PR, I used the gci linter via golangci-lint to do the reorganization which eliminates extra sections and groups like I think we want.

A lot of files have changed, but you should be able to follow the commits as so:

1. Update all pre-commit hooks; fix issues discovered by updates: this updates all of our pre-commit hooks (via pre-commit autoupdate) to their latest versions, then fixes a few new issues that popped up from the updated versions of golangci-lint and markdownlint. I needed the latest golangci-lint for gci to work properly.
2. Update nix config to use pre-commit 2.20: don't think that was required, but I noticed the nix config was several versions behind a brew'd version as well as what we use in the circleci-docker repository.
3. Removed some import duplication that was causing inconsistent import formatting by linter (and just confusing): fixed some import duplication and import comments that were causing gci to flip import order back and forth when run. I assume it was unintentional (although legal) to have the import duplication.
4. Add gci linter to golangci-lint to format order/grouping of go imports: added the gci linter and settings to golangci-lint and included the go files where gci fixed the import order (appears to be most of them).

To test it, try changing the order of imports on a file or two, do a git add on them, then run:
pre-commit run golangci-lint

To try it on all files (it shouldn't touch generated files),
pre-commit run -a golangci-lint

Special Notes

• one of the new linter issues I had to fix involved setting a ReadHeaderTimeout in addition to the existing IdleTimeout (gosec rule G112: Potential slowloris attack). Since it wasn't set previously, I think there was no timeout. I set it to 60 seconds (half of the 120 second idle timeout), but I could use some advice on if another value makes more sense.
• there is a performance problem when running the revive linter under golangci-lint in the latest 1.47.*+ versions. Temporarily, golangci-lint has turned off some revive rules as a workaround until it can be fixed upstream. I'm hoping this might be fixed by the time we would consider merging this, so I'm making this a draft PR for now. If not, we'll have to assess if it makes sense to upgrade now and have some rules turned off for a bit.
• the performance problem noted above has been fixed in revive and included in golangci-lint 1.49.0 (along with reversing the rules that had been temporarily turned off).
• the latest golangci-lint has deprecated a few linters that we use. Rather than complicate this PR any further, I'd prefer to assess the impact of those deprecations in a separate PR. We can also review our use of this workaround as well since some linters are more generics-friendly now.

[reggieriser]

Original linter issue: G112: Potential Slowloris Attack because ReadHeaderTimeout is not configured in the http.Server (gosec)

[reggieriser]

Original linter issue (several of these): SA1019: tlsConfig.BuildNameToCertificate has been deprecated since Go 1.14: NameToCertificate only allows associating a single certificate with a given name. Leave that field nil to let the library select the first compatible chain from Certificates. (staticcheck)

[reggieriser]

Example original linter issue (several in this file): MD053/link-image-reference-definitions Link and image reference definitions should be needed [Unused link or image reference definition: "contributors"] [Context: "[contributors]: https://github..."]

[robot-mymove]

	Messages
📖	🔗 MB-13510

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[LeDeep]

Played around with order of imports and pre-commit run -a golangci-lint was able to resolve them. Changes looks good. Updated links are all working. Tests pass and was able to run server without issue.