Remove WORKSPACE files

[nettle]

Why:
Bazel 8 disables the WORKSPACE by default and Bazel 9 removed it.
Our WORKSPACE file is not used FOSS tests were skipped on Bazel 8,
which left yaml-cpp untested there.

What:

• Delete WORKSPACE and test/foss/templates/WORKSPACE.template
• Point the buildifier at MODULE.bazel
• Remove zlib-module FOSS test
• Convert yaml-cpp FOSS test to bzlmod, pin tag yaml-cpp-0.9.0
• Drop the bazel 8 skip and the WORKSPACE handling from the FOSS tests
• Document only the MODULE.bazel

[furtib]

If we do not need to support WORKSPACE files, I'm all for this change!

Keep in mind that CI has not run for whatever reason!

I was hesitant to take this step due to Bazel 7 still supporting WORKSPACE files.
If we merge this don't forget to close: #258.

[nettle]

If we do not need to support WORKSPACE files, I'm all for this change!

Keep in mind that CI has not run for whatever reason!

I was hesitant to take this step due to Bazel 7 still supporting WORKSPACE files. If we merge this don't forget to close: #258.

I think WORKSPACE support might be still important for Bazel 7,
but in this case we need a proper test!

@furtib, @Szelethus, please let me know if it is OK to merge since CI checks seem broken.

[nettle]

... since CI checks seem broken.

CI checks were affected by GitHub incident (https://www.githubstatus.com/incidents/qcvjkzcs7j74) which is fixed now.
Checks were retriggered and passed.
Merging...