Revert "CSS: Remove uppercase transform on menu headings"

This reverts commit c1feb96ab3ee8ad7e6e4a7f1a0d986b763510ef3.
Revert "Move menu generation functionality into Menu Class and update CSS"

This commit was merged without approval or review from the team, and
introduced several regressions and issues, documented in

- Issue #17712: errors in menu and submenu display
- Issue #17779: New Menu Class is breaking plugins

See also issue #17711 and comments in pull request #316.

This reverts commit 29c2f8211de8869b3b1b072467b54252d0db5024.