Update comments and errors messages for: Replace centos:8 with almalinux:8

[simonmarty]

Description

[Describe what this change achieves]

Followup to comments on this commit to update error messages and comments.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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DCO Failure

[cwperks]

@simonmarty could you please sign your commits. See https://github.com/opensearch-project/OpenSearch/pull/19159/checks?check_run_id=49030173870 for details.

[cwperks]

Ignore the Changelog Verifier check. We can add a label for skip-changelog to mute that check.

[simonmarty]

Couple extra places that might/might not need changes:

• I don't see AlmaLinux here
• Not sure how much longer the CentOS docker artifact repo here will keep being updated.

[cwperks]

I don't see AlmaLinux here

Looks like that is in the ingest-user-agent module (public docs). Not sure of the purpose of that module, but I believe it helps with normalizing and extracting user agent from a REST Request so not directly related to the base image change but certainly would be good to verify that the regex is as exhaustive as can be.

Not sure how much longer the CentOS docker artifact repo here will keep being updated.

I also think that would be safe to remove.

[simonmarty]

On the second one, rpm_docker should be superseded by dnf_docker here anyways, which already points to the Fedora Docker artifacts. I'll leave these out of scope for this PR.

[github-actions]

✅ Gradle check result for df96018: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.98%. Comparing base (5b89711) to head (df96018).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19159      +/-   ##
============================================
+ Coverage     72.94%   72.98%   +0.03%     
- Complexity    69684    69687       +3     
============================================
  Files          5655     5655              
  Lines        319867   319867              
  Branches      46337    46337              
============================================
+ Hits         233342   233452     +110     
+ Misses        67638    67509     -129     
- Partials      18887    18906      +19     

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