cache v1 & v2 being binned

[rmbielby]

cache v1 and v2 are being scrapped and workflows need updating to use either v3 or v4.

[cjrace]

For simplicity / laziness moving forwards we can set up dependabot to track GitHub actions versions and automatically raise PRs to bump them when there are new versions. I have an example elsewhere - https://github.com/cjrace/almostyellow/blob/main/.github/dependabot.yml