Update GitHub actions/workflows

[abhro]

• Use dependabot to maintain GitHub actions
• Update action versions manually (bypasses dependabot PR)
• Instead of manually specifying julia 1.6 for testing, test on earliest supported version by using 'min'
• Test on newest julia v1
• Use shell option for easier to read steps in documenter.yml

[abhro]

As far as I can tell, CI failure based on newer Julia versions updating how they output particular objects, which this PR brings to light by adding the version '1' to the CI testing matrix. This CI failure should highlight that some code needs to be updated to comply with the regular Julia API.

[abhro]

soft ping @SimonDanisch

[SimonDanisch]

Thanks, merged in: #121