Summary:
This revision addresses all the remaining YB_TODOs in nodeModifyTable.c:

- The functionality of YbExecUpdateAct is deduplicated and merged into ExecUpdateAct. Variables introduced by YB are marked with a `yb_` prefix.
   - YB relations invoking ExecUpdateAct now return TM_Ok for row found and TM_Invisible for row not found.
- Call to always fetch the new slot tuple via `ExecFetchSlotHeapTuple` for YB relations is now skipped.
   - Functions in ybOptimizeModifyTable.c that previously required the newtuple via `(oldtuple, newtuple)` args now require `(oldtuple, newtupleslot)`.
   - This makes it consistent with ExecUpdate* function signatures in PG 15.
- Changes to ExecDelete between PG 15 and PG 11 were merged in an inconsistent fashion when compared to ExecUpdate. The following changes are made:
    - Tuple deletion logic for YB relations is moved into ExecDeleteAct.
    - Index deletion logic for YB relations is moved into ExecDeleteEpilogue (see ExecUpdateEpilogue for reference)
    - The return values of ExecDeleteAct have the same semantics as ExecUpdateAct.
Jira: DB-16665

Test Plan: Run Jenkins

Reviewers: aagrawal, amartsinchyk

Reviewed By: aagrawal

Subscribers: smishra, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D43632