CI: Reduce the number of CI jobs

[DilumAluthge]

Before this PR, we have 64 CI jobs, which is a lot.

This PR reduces the number of CI jobs:

1. For Linux 64-bit, we test against every Julia version.
2. For Linux 32-bit, we only test against Julia v1.
3. For Windows 64-bit, we only test against Julia LTS and Julia v1.
4. For Windows 32-bit, we only test against Julia v1.

After this PR, we have 23 CI jobs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.94%. Comparing base (5be878b) to head (0afe52e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #108   +/-   ##
=======================================
  Coverage   75.94%   75.94%           
=======================================
  Files          31       31           
  Lines         819      819           
=======================================
  Hits          622      622           
  Misses        197      197           

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[oxinabox]

Suggested change

	# To reduce the number of CI jobs, we test Linux 64-bit against all Julia versions (see above),
	# but for other platforms:
	# We test Linux 64-bit against all Julia versions (see above),
	# but for other platforms we test less (see below). Thus reducing number of CI jobs

[oxinabox]

I am not sure it's worth having CI on anything except linux if it doesn't test all versions.
Because a change that only effects one version could be made in any PR.