Changes

ci(tekton): support calendar-style nextgen tags (#4387)

## Summary
- accept calendar-style nextgen tags such as `v26.3.1` in the nextgen
tag-build trigger and downstream cloud delivery notifications
- keep plain semver nextgen versions unchanged on `release-nextgen-*`
branches and prefer `release-nextgen-*` when resolving containing
branches for tag-triggered builds
- extend the versioning strategy tests and docs with the new nextgen tag
style

## Testing
- `deno test scripts/flow/build/versioning-strategy.test.ts`
- `ruby -e 'require "yaml"; ...'` against the changed Tekton YAML files
- `git diff --check`

## Notes
- The new plain-semver nextgen tag detection in the tag trigger and Ops
notification path assumes the new nextgen tags use a 2+ digit major, for
example `v26.3.1`.
- `PingCAP-QE/artifacts` still has `delivery.yaml` / package delivery
rules that only match legacy nextgen tags today, so image/package
distribution for `v26.3.1` likely needs a separate follow-up there.
 (commit: 417be2f)