Add cron method to gc LFS MetaObjects (#22385)

This PR adds a task to the cron service to allow garbage collection of
LFS meta objects. As repositories may have a large number of
LFSMetaObjects, an updated column is added to this table and it is used
to perform a generational GC to attempt to reduce the amount of work.
(There may need to be a bit more work here but this is probably enough
for the moment.)

Fix #7045

Signed-off-by: Andrew Thornton <art27@cantab.net>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -1039,6 +1039,16 @@ Default templates for project boards:
 - `SCHEDULE`: **@every 168h**: Cron syntax to set how often to check.
 - `OLDER_THAN`: **@every 8760h**: any system notice older than this expression will be deleted from database.
 
+#### Cron -  Garbage collect LFS pointers in repositories ('cron.gc_lfs')
+
+- `ENABLED`: **false**: Enable service.
+- `RUN_AT_START`: **false**: Run tasks at start up time (if ENABLED).
+- `SCHEDULE`: **@every 24h**: Cron syntax to set how often to check.
+- `OLDER_THAN`: **168h**: Only attempt to garbage collect LFSMetaObjects older than this (default 7 days)
+- `LAST_UPDATED_MORE_THAN_AGO`: **72h**: Only attempt to garbage collect LFSMetaObjects that have not been attempted to be garbage collected for this long (default 3 days)
+- `NUMBER_TO_CHECK_PER_REPO`: **100**: Minimum number of stale LFSMetaObjects to check per repo. Set to `0` to always check all.
+- `PROPORTION_TO_CHECK_PER_REPO`: **0.6**: Check at least this proportion of LFSMetaObjects per repo. (This may cause all stale LFSMetaObjects to be checked.)
+
 ## Git (`git`)
 
 - `PATH`: **""**: The path of Git executable. If empty, Gitea searches through the PATH environment.