Bump yahnis-elsts/plugin-update-checker in /wordpress-plugin

Bumps [yahnis-elsts/plugin-update-checker](https://github.com/YahnisElsts/plugin-update-checker) from 5.3 to 5.4.
- [Release notes](https://github.com/YahnisElsts/plugin-update-checker/releases)
- [Commits](YahnisElsts/plugin-update-checker@v5.3...v5.4)

---
updated-dependencies:
- dependency-name: yahnis-elsts/plugin-update-checker
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>