chore(ci): rename self-host-nightly.yml to self-host-maintenance.yml (#8483)

The workflow has no schedule: trigger -- workflow_dispatch is its only one, and
its own header says that is deliberate while the review stack runs without
colocated runners. Its name: field already reads "self-host maintenance"; only
the filename still implied an unattended nightly job, which is misleading when
surveying .github/workflows/ by filename.

Pure git rename: no content, trigger, or job changes. Nothing else in the repo
referenced the old filename.

Closes #8397

Co-authored-by: RealDiligent <nft.gold.eth@gmail.com>

Author: RealDiligent