[TASK] Protect internal properties of PageRepository

Mark the public properties as protected and deprecate them within
PageRepository:

- workspaceCache
- error_getRootLine
- error_getRootLine_failPid

Change-Id: I4ed101794dcfc7fa054d42d9e58ad8b5718bcc5d
Releases: master
Resolves: #81318
Reviewed-on: https://review.typo3.org/52918


Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>