Update GitHub Actions CI

[striezel]

The following updates are performed:

• update actions/cache to v4
• update actions/checkout to v4

Still using the outdated actions will generate several warnings in CI runs, for example in https://github.com/boostorg/nowide/actions/runs/8432630230:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3, actions/cache@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

The PR will get rid of those warnings, because the newer versions of those actions use Node.js 20.

[Flamefire]

I'm afraid the container runs will fail as node 20 isn't available in Ubuntu 16 & 18, there is a workaround in Boost.CI: https://github.com/boostorg/boost-ci/blob/ed7ead8ae9b31b7f523850b0a0debd255a62f0cf/.github/workflows/ci.yml#L145-L147

[striezel]

OK. I've added the workaround. Let's see whether this works here, too.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (9d35cc4) to head (dd1a4f1).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #179      +/-   ##
===========================================
+ Coverage    97.72%   99.45%   +1.73%     
===========================================
  Files           34       34              
  Lines         3034     3319     +285     
===========================================
+ Hits          2965     3301     +336     
+ Misses          69       18      -51     

see 31 files with indirect coverage changes

[Flamefire]

Ok, makes the file and output a bit more ugly but hopefully can be removed at some point. Thanks!