Remove GH Action to validate OPTiMaDe instances

[CasperWA]

Closes #223

Use Materials-Consortia/optimade-validator-action instead in CI.

Also, use the new validator version to pass the sha of either the "merge commit", i.e., the latest head commit in a PR, no matter where the head is, or the sha of the latest push commit (since the CI workflow is also triggered when pushing to master).

[codecov]

Codecov Report

Merging #224 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   87.55%   87.55%           
=======================================
  Files          43       43           
  Lines        1912     1912           
=======================================
  Hits         1674     1674           
  Misses        238      238           

Flag	Coverage Δ
#unittests	87.55% <ø> (ø)

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[CasperWA]

Currently, the docker-image test is failing due to #225 not being merged.

[CasperWA]

@shyamd You can also incorporate this PR into your restructuring if you wish - keeping it a separate commit in the end?

[shyamd]

Yeah, I'm gonna close that PR and rebuild from scratch. Too many things are in flux.

[CasperWA]

By the way, this cannot be merged until #225 is merged.

[ml-evs]

Happy with this, nice job @CasperWA! Do you want to merge the outstanding action PR first so we have a v1(.1?) run of the action here?

[CasperWA]

Happy with this, nice job @CasperWA! Do you want to merge the outstanding action PR first so we have a v1(.1?) run of the action here?

Again, the idea is to always point to v1, and then just update that tag - which means the two application should be completely separate... well, as separate as one pointing to the other can be ... :)