change service account embedded policy size limit (#19840)

Bonus: trim-off all the unnecessary spaces to allow
for real 2048 characters in policies for STS handlers
and re-use the code in all STS handlers.

cmd/iam-store.go

@@ -2371,7 +2371,7 @@ func (store *IAMStoreSys) UpdateServiceAccount(ctx context.Context, accessKey st
 			return updatedAt, err
 		}
 
-		if len(policyBuf) > 2048 {
+		if len(policyBuf) > maxSVCSessionPolicySize {
 			return updatedAt, errSessionPolicyTooLarge
 		}