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