Update CI workflows including cutting Python v3.10 support

[sadielbartholomew]

Close #964 (next release will be after the October v3.10 EOL), close #969 whilst updating the CI/Actions workflows (which could do with many bumps on the external Actions versions) and close #966.

[sadielbartholomew]

Closing and re-opening to refresh the CI jobs matrix...

[sadielbartholomew]

Another open-close for CI job refresh - seems to pick up the edited workflows whereas clicking the 're-run jobs' buttons doesn't sometimes...

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@c906bb0). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #967   +/-   ##
=======================================
  Coverage        ?   82.90%           
=======================================
  Files           ?      105           
  Lines           ?    17123           
  Branches        ?        0           
=======================================
  Hits            ?    14194           
  Misses          ?     2929           
  Partials        ?        0           

Flag	Coverage Δ
unittests	82.90% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sadielbartholomew]

OK the Ubuntu CI jobs are now passing, finally. I commented out the latest Mac OS as an <job>.strategy.matrix.os option since I suspect there is a separate issue there causing all jobs to fail early, but I am now adding that back in to see. There is a further issue with the test-source-dist.yml workflow but I think that is close to passing now. Let's see...

[sadielbartholomew]

test-source-dist.yml workflow now also passing but there is an early failure with the Mac OS case. Trying again...

[sadielbartholomew]

Now all running but the Mac OS and Ubuntu runs seem to have picked up on some genuine issues (possible bugs), so noting those and investigating separately. Now adding in missing optional dependencies to the workflows so we don't have any avoidable test skipping on the jobs.

[sadielbartholomew]

One last time to run the jobs after pushing a fix for one trivial issue raised by one particular job, and then I am satisfied to merge because the remaining failures are genuine issues raised (see #968, NCAS-CMS/cfdm#429).

[sadielbartholomew]

OK I am satisfied this PR has achieved its aims now, namely to get the CI workflows up-to-date and passing (excluding failures due to written-up follow-on issues, see below); remove Python 3.10 support; and replace the deprecated build command with the recommended modern equivalent (and also remove the etc dir. which we realised is long unused).

Though it was more encompassing than I thought originally since the CI jobs showcased various issues we were as yet unaware of. So apologies for some bloat instead of multiple PRs, it just made it easier to check on the state of the CI jobs. I have fixed anything trivial and put up Issues for the test:

• Attempt at unsupported conservative 3D mesh regridding #968 which is causing the Linux (3.14 at least, maybe all other Linux jobs which cancel on one failure) job to fail;
• #to_xarray for time coords beyond np.datetime64[ns] cfdm#429 which is causing the Mac OS 3.13 test suite and test source dist jobs (at least, maybe the whole Mac OS job set) to fail.

@davidhassell please let me know if you prefer to re-review since there have been numerous commits since you last reviewed, else if you're happy for me to merge.