ingest/ledgerbackend: Fix captive core toml history entries

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Remove trailing slash from history archive urls when generating the captive core toml file.

Why

If you ran the horizon db reingest range command with a NETWORK configured to pubnet, the command would fail because the history archive urls we use all have trailing slashes:

https://github.com/stellar/go/blob/master/network/main.go#L26-L28

This would result in core making curl requests to invalid urls.

Known limitations

[N/A]

[urvisavla]

Considering whether removing the trailing slash here is a better option as it seems to be used in this?

[tamirms]

@urvisavla history archive urls can be configured via command line. So, even if we strip the trailing slash in https://github.com/stellar/go/blob/master/network/main.go#L26-L28 , it is still possible for horizon to fail a reingest command because the history archive urls provided by the user contain a trailing slash