chore: disable broken auto-bump workflow

bump-plugin-version.yml has been failing on every merge to main since
today's security + plugin-packaging work, because it tries to push
directly to main and branch protection blocks it. It also conflicts
with the manual version-management pattern we're currently using
(manual bumps in PRs like #409 for 3.1.0).

Renaming to .yml.disabled so GitHub Actions skips it. If we want
auto-bumps later, the workflow needs to open a PR instead of pushing
directly, and coordinate with manual version bumps.

Author: web-flow