Use ExecComputeStoredGeneratedCompat macro

[svenklemm]

When adding support for PG13 we introduced a macro to hide the
function signature differences of ExecComputeStoredGenerated
but the callers of this function never got adjusted to use the
macro.

[codecov]

Codecov Report

Merging #3301 (99a19e5) into master (5719c50) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3301      +/-   ##
==========================================
- Coverage   90.51%   90.48%   -0.03%     
==========================================
  Files         212      212              
  Lines       35494    35444      -50     
==========================================
- Hits        32128    32073      -55     
- Misses       3366     3371       +5     

Impacted Files	Coverage Δ
src/copy.c	90.61% <100.00%> (+0.17%)	⬆️
tsl/src/nodes/data_node_copy.c	95.58% <100.00%> (ø)
tsl/src/nodes/data_node_dispatch.c	97.08% <100.00%> (-0.25%)	⬇️
src/loader/bgw_message_queue.c	85.52% <0.00%> (-2.64%)	⬇️
src/import/planner.c	68.96% <0.00%> (-0.77%)	⬇️
src/chunk_append/explain.c	82.53% <0.00%> (-0.28%)	⬇️
src/plan_expand_hypertable.c	93.75% <0.00%> (-0.27%)	⬇️
src/chunk_append/transform.c	96.42% <0.00%> (-0.24%)	⬇️
src/bgw/scheduler.c	82.89% <0.00%> (-0.16%)	⬇️
... and 12 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8e1273c...99a19e5. Read the comment docs.

[mkindahl]

LGTM. Checked if there were other missing locations, but chunk_dispatch_state.c was already correct.