disallow delete of only SpecRevision

[theganyo]

Fixes #257

[codecov[bot]]

Codecov Report

Merging #698 (7970956) into main (8029db1) will increase coverage by 0.03%. The diff coverage is 100.00%.

:exclamation: Current head 7970956 differs from pull request most recent head 4de68ab. Consider uploading reports for the commit 4de68ab to get more accurate results

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
+ Coverage   55.49%   55.53%   +0.03%     
==========================================
  Files          93       93              
  Lines        7952     7952              
==========================================
+ Hits         4413     4416       +3     
+ Misses       3071     3069       -2     
+ Partials      468      467       -1     

Impacted Files	Coverage Δ
server/registry/actions_spec_revisions.go	38.16% <100.00%> (+2.29%)	:arrow_up:

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