Revert "feat: drop node target's key constraint requirement (#2068)"

[fbiville]

This reverts commit 6804d64.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 46.61%. Comparing base (f933ae9) to head (9ad8cc8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rt/v2/neo4j/model/validation/NodeKeyValidator.java	78.57%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2131      +/-   ##
============================================
- Coverage     46.62%   46.61%   -0.01%     
- Complexity     3953     3954       +1     
============================================
  Files           867      868       +1     
  Lines         51567    51581      +14     
  Branches       5399     5401       +2     
============================================
+ Hits          24043    24047       +4     
- Misses        25808    25814       +6     
- Partials       1716     1720       +4     

Components	Coverage Δ
spanner-templates	68.60% <ø> (-0.03%)	⬇️
spanner-import-export	65.54% <ø> (-0.08%)	⬇️
spanner-live-forward-migration	77.49% <ø> (ø)
spanner-live-reverse-replication	78.43% <ø> (ø)
spanner-bulk-migration	87.66% <ø> (ø)

Files with missing lines	Coverage Δ
...rt/v2/neo4j/model/validation/NodeKeyValidator.java	78.57% <78.57%> (ø)

... and 3 files with indirect coverage changes

[fbiville]

Unrelated modules' tests fail in CI.