gitaly: Upgrade server version to include atomic repo creation

Upgrade the Gitaly server version to f87bc1e98 (Merge branch
'pks-create-repository-atomic' into 'master', 2021-11-24), which
introduces a feature flag into Gitaly's code base changing the semantics
of CreateRepository and related RPCs to create repositories atomically.
This both implements proper voting on created repositories in Gitaly
Cluster setups, and causes these RPCs to fail in case the target
repository exists already.

GITALY_SERVER_VERSION

-518670d57d1a6527aaf46b5b9bf5cb00f2e8f11b
+f87bc1e983d11788fdbce953dced45ec5554af23