[datadog-operator] Update Github Workflow for Milestone

[gabedos]

What does this PR do?

Update the Github Workflows surrounding PR Milestones.

• Decouples the qa/skip-qa label from the Milestone. Targeting a particular release should be independent of whether or not QA is required. By default, QA remains automatically required and only familiar Operator users will add the label to skip QA.

• Every PR will have a milestone associated to it determine on the base branch that it merged onto to. (Merge onto main gets latest Milestone, Merge onto rc branch gets the rc milestone). This mirrors the datadog-agent milestone workflow which adds a milestone post merge.

Describe your test plan

The add-milestone action was mostly copied from the Agent. The main difference is how we're querying for the milestone itself. The Agent stores the active milestone explicitly within a file in the code base. Theoretically we could do the same by looking at config/manager/kustomization.yaml for the newTag field. However, @claude decided to instead directly query the open milestones within the repo and pick the latest when merging onto main. Testing the commands locally seem good:

Getting the Milestone when merging onto main:

gh api repos/datadog/datadog-operator/milestones --jq '[.[] | select(.state == "open")] | sort_by(.number) | last | .title'

Getting the Milestone when not merging onto main:

 bash -c '                                                                                                                                                            
  BASE_BRANCH="v1.25"                                                                                                                                                  
  if [[ "$BASE_BRANCH" =~ ^v([0-9]+)\.([0-9]+)$ ]]; then                                                                                                               
    echo "Full match: ${BASH_REMATCH[0]}"
    echo "Major: ${BASH_REMATCH[1]}"                                                                                                                                   
    echo "Minor: ${BASH_REMATCH[2]}"                                                                                                                                   
    echo "Milestone: v${BASH_REMATCH[1]}.${BASH_REMATCH[2]}.0"                                                                                                         
  fi'

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• All commits are signed (see: signing commits)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.94%. Comparing base (a1206ff) to head (eb0c76c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2837   +/-   ##
=======================================
  Coverage   38.94%   38.94%           
=======================================
  Files         313      313           
  Lines       27139    27139           
=======================================
  Hits        10570    10570           
  Misses      15780    15780           
  Partials      789      789           

Flag	Coverage Δ
unittests	38.94% <ø> (ø)

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