Mark execution mode enum reserved

[katrogan]

Why are the changes needed?

What changes were proposed in this pull request?

see PR title

How was this patch tested?

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.04%. Comparing base (6f187ae) to head (6dbd5d9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6040      +/-   ##
==========================================
- Coverage   37.05%   37.04%   -0.01%     
==========================================
  Files        1316     1316              
  Lines      132247   132247              
==========================================
- Hits        48998    48991       -7     
- Misses      78987    78994       +7     
  Partials     4262     4262              

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.07% <ø> (-0.03%)	⬇️
unittests-flytecopilot	22.23% <ø> (ø)
unittests-flytectl	62.46% <ø> (ø)
unittests-flyteidl	7.24% <ø> (ø)
unittests-flyteplugins	53.73% <ø> (ø)
unittests-flytepropeller	42.63% <ø> (ø)
unittests-flytestdlib	57.57% <ø> (-0.02%)	⬇️

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