Execute workloads based on labels instead of flags in PR titles

[kate-goldenring]

Currently, certain workflows are being skipped/executed based on flags set in PR titles. For example, adding [SAME VERSION] to a PR title prevents the version check workflow from running. However, after adding a flag to a title, checks cannot simply be re-run, since they refer to the state of the PR of the last commit. Resultingly, another commit must be pushed to the PR in order for checks to properly register the PR title change (and therefore flag). Running workflows based on labels instead of flags may remove this issue of pushing new commits.

Work to be done:

1. Confirm whether actions can reference newly added labels without new commits needing to be pushed. (Even if this isnt the case, labels are still cleaner than PR title flags)
2. In the opencv base build, rust base build, and version check workflows, github.event.pull_request.title should be changed to github.event.label.name and point to newly made labels (possibly same version, allow intermediate builds, and ignore intermediate builds).
3. Update PR template and contributing doc to explain this new model

[vincepnguyen]

I will investigate this issue

[github-actions]

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