Revert logical snapshot filename format change in SnapBuildSnapshotExists().

Commit 2633dae2e4 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

src/backend/replication/logical/snapbuild.c

@@ -2061,7 +2061,7 @@ SnapBuildSnapshotExists(XLogRecPtr lsn)
 	int			ret;
 	struct stat stat_buf;
 
-	sprintf(path, "%s/%08X-%08X.snap",
+	sprintf(path, "%s/%X-%X.snap",
 			PG_LOGICAL_SNAPSHOTS_DIR,
 			LSN_FORMAT_ARGS(lsn));