backup: clean up fat manifest support

Since #93997 we have written a file.sst, descriptor.sst and BACKUP_METADATA file. The intent was to to remove support for the "fat" BACKUP_MANIFEST file. There are still many read paths, like show backup, that only work if the fat manifest is present.

Here's the current plan to delete the fat manifest:
- starting in 25.4 the slim manifest will always get written (previously, the write could have been skipped if default setting was turned off)
- in 25.4, there are still 2 read paths that rely on the fat manifest: LegacyFindPriorBackups and ListFullBackupsInCollection. Both of these are fallback paths if the user does not want to read with the new backup index. Reads from the backup index were introduced in 25.4, and will be turned default on in 26.2.
- By 26.4, we can remove the index cluster setting, and consequently, remove the legacy read paths that rely on the fat manifest. Once the read paths are deleted, we can remove the write path.

We could have added extra logic to LegacyFindPriorBackups and ListFullBackupsInCollection to read with the new manifests, but that seemed overly complicated, and required dealing with dupes. Perhaps we can revisit this in 26.2, when the new manifest will always get written.

Jira issue: CRDB-46527