Bump versions of GitHub Actions

[penelopeysm]

This PR bumps the versions of things like actions/checkout to the latest. Not hugely important, it just suppresses a couple of warnings on CI runs.

[coveralls]

Pull Request Test Coverage Report for Build 11148586360

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.129%

---

Totals
Change from base Build 11148353117:	0.0%
Covered Lines:	2790
Relevant Lines:	3571

---

💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.53%. Comparing base (4c60c9f) to head (bfb41be).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
- Coverage   76.62%   75.53%   -1.09%     
==========================================
  Files          29       29              
  Lines        3589     3323     -266     
==========================================
- Hits         2750     2510     -240     
+ Misses        839      813      -26     

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[devmotion]

Might be a good opportunity to add dependabot, see eg https://github.com/JuliaStats/StatsFuns.jl/blob/master/.github/dependabot.yml.