[confmap] Set confmap.unifyEnvVarExpansion feature gate to stable

[TylerHelmuth]

Description

This PR promotes the confmap.unifyEnvVarExpansion feature gate to stable and sets a ToVersion of v0.106.0, anticipating that the gate be completely removed in that version.

We should weigh if switching the Stable should be done in v0.105.0 or if it needs more time in Beta to give users more time to switch. Delaying promotion to Stable delays confmap 1.0.

If we merge this we need to commit to merging #10510 in the same release.

Link to tracking issue

Related to #10161
Related to #7111
Related to #8215

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.37%. Comparing base (420bf36) to head (bebb922).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10508      +/-   ##
==========================================
- Coverage   91.64%   91.37%   -0.27%     
==========================================
  Files         406      406              
  Lines       18985    19007      +22     
==========================================
- Hits        17398    17368      -30     
- Misses       1227     1282      +55     
+ Partials      360      357       -3     

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[TylerHelmuth]

Making the feature gate stable negates these tests, which set the gate to false. Since the module is being deprecated, and will be removed shortly after, removing the tests doesn't feel like a big deal.

[TylerHelmuth]

Let me know if you'd rather this PR and #10510 be merged into 1. If we needed to roll back for some reason 1 PR would be easier.

[mx-psi]

I think we can wait at least until #10554 has been out for one release

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[ben-childs-docusign]

Hi Folks - I just logged this issue it seems like there is still an issue with configurations that use "$$SomeString" syntax to represent the string literal "$SomeString" with this feature flag enabled it is still blocked on startup with version 105.

#10713

[mx-psi]

@ben-childs-docusign Thanks for the report. Have you tried v0.106.1? The fix for that issue was released on v0.106.0

[ben-childs-docusign]

Hi @mx-psi yes we deployed the patch and it is working. thanks for the fix!

[mx-psi]

I am going to wait to merge this until #10510 is approved