Re: When deleting the plpgsql function, release the CachedPlan of the function

This patch only addresses the plan cache release for plpgsql (the procedural language) and does not cover SQL functions.

Since the src/backend/utils/cache/funccache.c file was introduced in PostgreSQL 18, I will first upgrade to version 18, 

then figure out how to extend the benefit to SQL functions. 

Additionally, I’m not entirely certain whether this patch’s modification is reasonable, or if it could lead to performance degradation. 

I might need everyone’s help to review it.