Gemfile: Bump gitaly version to v13.11.0.pre.rc1

Gitaly has recently introduced two new RPCs `ListLFSPointers` and
`ListAllLFSPointers` which are slated to replace the current RPCs
`GetLFSPointers`, `GetNewLFSPointers` and `GetAllLFSPointers`.

As a preparatory step to make use of the replacements, this commit
updates the Gitaly Gem to v13.11.0.pre.rc1.