Update CI with new MATLAB version (2023b) and remove 2021a tests

[RichardWaiteSTFC]

We believe the failure of the build_ctf on the ESS self-hosted runner (mac) is because MATLAB 2023b is not installed

[github-actions[bot]]

Test Results

       4 files  ±0     106 suites  ±0   14m 48s :stopwatch: - 3m 14s    619 tests ±0     618 :heavy_check_mark:  - 1  0 :zzz: ±0  1 :x: +1  1 766 runs  ±0  1 765 :heavy_check_mark:  - 1  0 :zzz: ±0  1 :x: +1 

For more details on these failures, see this check.

Results for commit 3ec0cd04. ± Comparison against base commit ea5ea334.

:recycle: This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ea5ea33) 40.48% compared to head (335b96b) 40.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #154 +/- ## ========================================== + Coverage 40.48% 40.51% +0.03% ========================================== Files 240 240 Lines 15979 15979 ========================================== + Hits 6469 6474 +5 + Misses 9510 9505 -5 ``` [see 4 files with indirect coverage changes](https://app.codecov.io/gh/SpinW/spinw/pull/154/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SpinW)

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[RichardWaiteSTFC]

Out of date now