[YSQL] [Concurrent DDL] Re-introduce yb_enable_concurrent_ddl GUC as no-op to prevent upgrade errors

[pao214]

Context

#31225 removed the yb_enable_concurrent_ddl GUC in favor of the gflag ysql_enable_concurrent_ddl. Clusters with the old GUC set in postgresql.conf will hit an unrecognized parameter error on upgrade.

Solution

Re-introduce yb_enable_concurrent_ddl as a deprecated no-op GUC that accepts the setting without error and emits a WARNING pointing to the correct gflag ysql_enable_concurrent_ddl.

References

• [YSQL] [Concurrent DDL] ysql_enable_concurrent_ddl gflag and require object locking to enable concurrent DDL #31225