CI: Add FreeBSD 14.0 and drop FreeBSD 12.4

[emaste]

FreeBSD 14.0 was released recently. FreeBSD 12.4 is EOL at the end of 2023.

Description

FreeBSD 14.0 added to Cirrus-CI. FreeBSD 12.4 removed from Cirrus-CI.

Checklist

• I have read followed CONTRIBUTING.md
  • I have added tests
  • I have updated the documentation
  • I have updated the CHANGELOG
• Are there any breaking changes?
  • If yes: I have marked them in the CHANGELOG (example)
• Does this PR introduce any platform specific code?
• Security: Does this PR potentially affect security?
• Performance: Does this PR potentially affect performance?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (34667f6) 100.00% compared to head (648badb) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #304   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         1662      1662           
=========================================
  Hits          1662      1662           

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[PJK]

It looks like that image is not available yet. How about freebsd-14-0-snap instead?