Epic: Improve feature versioning

[JeromeJu]

This epic tracks the subtasks on improving Tekton pipeline feature versioning including feature flag improvements, documentations, maintenance routine and cleanups etc.

🎏 Feature flag improvements

• Per feature flag
• Decoupling API versioning and Feature versioning for features turned on by default
• Default enable-api-fields value for opt-in features once feature and API versioning are decoupled
• TEP0138 Per-feature Flags Improvement to Decouple API and Feature Versioning #7177

📑 Documentations

• Update our policies/development guide/TEP process to mention that new features start out in the alpha track
• Update our policies to mention that new feature authors should open an issue to track the feedback on that feature and promotion to beta once the feature lands in a release
• Better documentations for feature stability levels

📝 Maintenance routine

• Consider retroactively opening issues to track feedback/promotion for current features in alpha
• Implement a quarterly alpha feature review to come up to review current features in alpha (this does not preclude moving features to beta outside of this review. This is mostly a way to ensure all alpha features are reviewed on a regular basis). First one schedule for September 12 during the API WG.

🧹 Cleanup

• Do not require enable-api-fields=alpha for spire
• Move feature flags testing functions into testing package
• Replace redundant function CheckAlphaOrBetaFields
• Update Stability Level for CSI and Projected Volume Workspaces

[tekton-robot]

@JeromeJu: The label(s) kind/epic cannot be applied, because the repository doesn't have them.

In response to this:

/kind epic

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[tekton-robot]

@JeromeJu: The label(s) kind/epic cannot be applied, because the repository doesn't have them.

In response to this:

/kind Epic

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[vdemeester]

/area epic

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[vdemeester]

/lifecycle frozen

[JeromeJu]

Closing with the completion of the following: #5632, #6592 and #6948