Simplify the release process (no more build script or release branches)

[DilumAluthge]

Fixes #280

After this PR, the release process of this repo becomes very simple:

1. Create the new vMAJOR.MINOR.PATCH tag locally (and push it).
2. Update the vMAJOR tag locally (and force-push it).
3. Go to https://github.com/julia-actions/setup-julia/releases and create the release.

This is the exact same release process that is used in the julia-actions/cache repo¹.

So, by having the same release process across multiple repos, this should make things easier for maintainers.

Benefits:

1. There are no more long release instructions.
2. There is no more build script that needs to be run.
3. There are no more release branches.
4. There is no need to run tests on the release branches, and then wait for those tests to pass before you can proceed.

---

Other benefits

1. As an added benefit, after this, PRs to this repo become useable. So, if someone wants to test out a PR before it is merged, they can simply do uses: julia-actions/setup-julia@name-of-pr-branch.

Footnotes

1. See the devdocs here: https://github.com/julia-actions/cache/blob/main/devdocs/making_a_new_release.md ↩

[omus]

I agree with the idea of checking in the built version

[DilumAluthge]

@omus This is ready for another round of review.

[DilumAluthge]

Bump @omus @IanButterworth

[IanButterworth]

If you believe in this design I'm happy to move to it. I suggest you lead the next release so that if anything goes wrong we can be reactive in fixing it.

[DilumAluthge]

I just cut a new release (v2.5.0), and I updated the v2 alias, just so that we can test out the new release process.

We can quickly revert if something breaks.

[DilumAluthge]

I also updated the latest alias.

[DilumAluthge]

From a quick test (JuliaLang/Example.jl#80), everything seems to be working.