Revert logical snapshot filename format change in SnapBuildSnapshotExists().

Revert logical snapshot filename format change in SnapBuildSnapshotExists().

Commit 2633dae2e48 standardized LSN formatting but mistakenly changed
the logical snapshot filename format in SnapBuildSnapshotExists() from
"%X-%X.snap" to "%08X-%08X.snap". Other code still used the original
"%X-%X.snap" format, causing the replication slot synchronization worker
to fail to find existing snapshot files and produce excessive log messages.

This commit restores the original "%X-%X.snap" format
in SnapBuildSnapshotExists() to resolve the issue.

Author: Shveta Malik <shveta.malik@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwHuHPB-ucAk_Tq3uSs4Fdziu1Jp_AA_RD3m5Ycky7m48w@mail.gmail.com