Changes

chore(prow-jobs/pingcap/docs): remove v5.4 as it is archived (#4548)

### What changed?
- remove the `release-5.4` postsubmit branch rule from
`prow-jobs/pingcap/docs/docs-cn-postsubmits.yaml`
- remove the `release-5.4` postsubmit branch rule from
`prow-jobs/pingcap/docs/docs-postsubmits.yaml`

### Why?
- `release-5.4` documentation has been archived and should no longer
trigger these docs postsubmit jobs

### Validation
- verified the diff only removes the `- ^release-5\.4$` rule from the
two target files
- loaded both YAML files successfully with Ruby `YAML.load_file`

### Reference
- https://github.com/PingCAP-QE/ci/pull/3638/files
- https://github.com/pingcap/docs-staging/pull/133
- https://github.com/pingcap/docs-staging/pull/134
 (commit: 43ee462)