[yugabyte#24954] YSQL: Fix duplicate GUC flag mask values

Summary:
The recent pg15 merge into master introduced two new flags within guc.h:
`GUC_EXPLAIN  0x100000`
`GUC_RUNTIME_COMPUTED  0x200000`

Prior to this merge, to help facilitate stickiness for custom string variables in Connection Manager, we had introduced a new flag:
`GUC_YB_CUSTOM_STICKY  0x100000`

Some GUC variables marked as GUC_EXPLAIN would be mistaken as "sticky" GUC variables due to having the same flag mask, this patch changes the two masks to be unique again:
`GUC_YB_CUSTOM_STICKY  0x400000`

Jira: DB-14091

Test Plan: Jenkins: enable connection manager, all tests

Reviewers: skumar, stiwary, mkumar, devansh.saxena, vpatibandla

Reviewed By: devansh.saxena

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D40051

Author: rahulb-yb

src/postgres/src/include/utils/guc.h

@@ -245,7 +245,7 @@ typedef enum
 
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
-#define GUC_YB_CUSTOM_STICKY	0x100000 /* stickiness for custom string
+#define GUC_YB_CUSTOM_STICKY	0x400000 /* stickiness for custom string
 										  * variables */