Add chunk exclusion for UPDATE for PG14

[svenklemm]

Currently only IMMUTABLE constraints will exclude chunks from an UPDATE plan,
with this patch STABLE expressions will be used to exclude chunks as well.
This is a big performance improvement as chunks not matching partitioning
column constraints don't have to be scanned for UPDATEs.
Since the codepath for UPDATE is different for PG < 14 this patch only adds
the optimization for PG14.

With this patch the plan for UPDATE on hypertables looks like this:

 Custom Scan (HypertableModify) (actual rows=0 loops=1)
   ->  Update on public.metrics_int2 (actual rows=0 loops=1)
         Update on public.metrics_int2 metrics_int2_1
         Update on _timescaledb_internal._hyper_1_1_chunk metrics_int2
         Update on _timescaledb_internal._hyper_1_2_chunk metrics_int2
         Update on _timescaledb_internal._hyper_1_3_chunk metrics_int2
         ->  Custom Scan (ChunkAppend) on public.metrics_int2 (actual rows=0 loops=1)
               Output: '123'::text, metrics_int2.tableoid, metrics_int2.ctid
               Startup Exclusion: true
               Runtime Exclusion: false
               Chunks excluded during startup: 3
               ->  Seq Scan on public.metrics_int2 metrics_int2_1 (actual rows=0 loops=1)
                     Output: metrics_int2_1.tableoid, metrics_int2_1.ctid
                     Filter: (metrics_int2_1."time" = length(version()))

[codecov]

Codecov Report

Merging #4209 (83dc529) into main (ff945a7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #4209   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         215      215           
  Lines       39530    39535    +5     
=======================================
+ Hits        35892    35897    +5     
  Misses       3638     3638           

Impacted Files	Coverage Δ
src/nodes/hypertable_modify.c	70.10% <100.00%> (+0.51%)	⬆️
src/planner.c	95.19% <100.00%> (+0.01%)	⬆️
src/bgw/scheduler.c	83.04% <0.00%> (-0.88%)	⬇️

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[gayyappan]

Changes look good to me. Minor comment.