ci: disable upstream Post-Merge Version Bump workflow on fork

Upstream's `post-merge-pr.yml` auto-creates a "chore: bump version to
vX.Y.Z" PR on every push to `main`, targeting the top-level VERSION /
package.json semver cadence. The fork bumps the plugin manifests
(plugin.json + marketplace.json) by hand and uses GitHub releases
instead, so this workflow only produces noise.

Symptoms before disable:
- The Create Pull Request step fails with `'version-bump' not found`
  because the fork's label set doesn't include `version-bump`.
- Each push leaves a stale `chore/version-bump-YYYYMMDD-HHMMSS` branch
  on origin (six accumulated already).

Renaming to `.yml.disabled` matches the precedent set by
`post-merge.yml.disabled` and keeps the file checked in for easy
re-enable if the fork ever wants the upstream cadence.

Author: MomePP