Introduce "job" version of prepare-pipelines.yml, with "pool" defined to use Ubuntu 20.04

[konrad-jamrozik]

This PR, in tandem with PRs #4915 and #4916, addresses #4888. This PR subsumes the abandoned PR of #4911.

Specifically, this PR ensures that prepare-pipelines.yml is configured to use specific pool, set to use Ubuntu 20.04:

pool:
  name: azsdk-pool-mms-ubuntu-2004-general
  vmImage: MMSUbuntu20.04

Without the pool definition pointing to Ubuntu 20.04, the steps within the file used an implicit default, which resolves to ubuntu-latest. This caused breakage as explained in #4888. Setting the pool to Ubuntu 20.04 will allow us to:

• Immediately unblock the language repos prepare-pipelines.yml pipelines [1], as Ubuntu 20.04 has the .NET Core version used by it [2];
• Allow us to migrate our tooling to .NET 6 as both Ubuntu 20.04 and ubuntu-latest, which is Ubuntu 22.04 [3], have .NET on them [4]. This step is accomplished by PRs Bump globals.yml .NET SDK version from 3.1.405 to 6.x + assorted changes #4916 and Update PipelineGenerator target framework from netcoreapp3.1 to net6.0 #4915;
• Allow us to migrate to Ubuntu 22.04 once we migrate to .NET 6, per issue Migrate all Ubuntu images in azure-sdk-tools (including latest) to Ubuntu 22.04 #4967.

Technical considerations of the changes

• The current prepare-pipelines.yml has only steps definition, which does not support pool definition. One needs to wrap the steps in job definition to support pool [5].
• Because the .yml file is located in eng/common/pipelines/templates/steps/ directory (note the steps), we cannot wrap the steps in job and leave it there; the introduction of job necessitates introducing the changes in eng/common/pipelines/templates/jobs/ directory instead (note the jobs), which is done in this PR.
• Because this PR introduces a new file, once this PR is merged, the language-specific prepare-pipelines.yml language pipelines need to be rewired to point to the newly introduced file. This approach was proposed in Add pool definition to prepare-pipelines.yml set to Ubuntu 20.04, and wrap the steps in a job #4911 (comment).
  • Language repos PRs doing the rewiring:
    Update prepare-pipelines.yml to use the new jobs-based template azure-sdk-for-net#32999
    ... more to come!

Testing done

I have confirmed the proposed modification to the .yml file will work by modifying the existing prepare-pipelines.yml directly to have exactly the same contents as the /jobs/prepare-pipelines.yml introduced in this PR, and observing the relevant build succeeds on azure-sdk-for-java:

while it failed before this change:

Footnotes

[1] for example, internal / java / prepare-pipelines pipeline, or internal / net / prepare-pipelines pipeline.

[2] as evidenced by the Ubuntu 20.04 image software page, it has .NET Core 3.1. The pool definition value points to a 1ES hosted image which has the software listed on that page, as explained by this comment: #4888 (comment).

[3] See this comment: #4888 (comment).

[4] as evidenced by the Ubuntu 22.04 image software page.

[5] Observe that on pool definition YAML reference page, we can read:

Properties that use this definition: pipeline.pool, stages.stage.pool, jobs.job.pool, jobs.deployment.pool.

i.e. steps.pool is not listed.

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[konrad-jamrozik]

/check-enforcer override

[konrad-jamrozik]

I have approved all releases from #4930 (comment)

[ghost]

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