server: fix bottlenecks when purging system.web_sessions

The previous purging limit was found to be too low which resulted in the
`system.web_sessions` table growing rapidly. Additionally, we were using
`ORDER BY random()` when deleting expired sessions from the table which
caused a full table scan of a large table. This resulted in high cpu
usage on every purge attempt with only very few rows actually being
deleted. Over time, this turns into a nonideal feedback loop.

This change fixes the two issues by:
1. Increasing the defauilt purging limit to 1000 from 10.
2. Remove the `ORDER BY random()` when purging the table to avoid full
   table scans on every purge.

support ticket: cockroachlabs/support#1812

resolves cockroachdb#88622

Release note: None

pkg/server/purge_auth_session.go

@@ -50,7 +50,7 @@ var (
 		settings.TenantWritable,
 		"server.web_session.purge.max_deletions_per_cycle",
 		"the maximum number of old sessions to delete for each purge",
-		10,
+		1000,
 	).WithPublic()
 )
 
@@ -90,21 +90,18 @@ func (s *authenticationServer) purgeOldSessions(ctx context.Context) {
 		deleteOldExpiredSessionsStmt = `
 DELETE FROM system.web_sessions
 WHERE "expiresAt" < $1
-ORDER BY random()
 LIMIT $2
 RETURNING 1
 `
 		deleteOldRevokedSessionsStmt = `
 DELETE FROM system.web_sessions
 WHERE "revokedAt" < $1
-ORDER BY random()
 LIMIT $2
 RETURNING 1
 `
 		deleteSessionsAutoLogoutStmt = `
 DELETE FROM system.web_sessions
 WHERE "lastUsedAt" < $1
-ORDER BY random()
 LIMIT $2
 RETURNING 1
 `