fix: Rollout's selector field should be immutable

[chetan-rns]

Fixes: #2105 #1761

Add a CEL validation to:

• Make the selector field required
• Make the selector field immutable.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[github-actions]

Go Published Test Results

2 164 tests   2 163 ✅  2m 54s ⏱️
  119 suites      0 💤
    1 files        1 ❌

For more details on these failures, see this check.

Results for commit e1437c4.

♻️ This comment has been updated with latest results.

[github-actions]

E2E Tests Published Test Results

  4 files    4 suites   3h 23m 17s ⏱️
111 tests  95 ✅  6 💤  10 ❌
614 runs  400 ✅ 24 💤 190 ❌

For more details on these failures, see this check.

Results for commit e1437c4.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.26%. Comparing base (f656735) to head (6a1a1a2).

❗ Current head 6a1a1a2 differs from pull request most recent head e1437c4

Please upload reports for the commit e1437c4 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3626      +/-   ##
==========================================
- Coverage   80.27%   80.26%   -0.01%     
==========================================
  Files         156      156              
  Lines       17966    17969       +3     
==========================================
+ Hits        14422    14423       +1     
- Misses       2631     2634       +3     
+ Partials      913      912       -1     

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
6.8% Duplication on New Code

See analysis details on SonarCloud