Use specific go versions in workflows and go.mo

[ineiti]

The current workflows all have the following:

      - name: Set up Go ^1.20
        uses: actions/setup-go@v3
        with:
          go-version: ^1.20

Or version 1.13. But this will always install the latest version of go, as of May 2024 this is 1.22.

I propose to choose an explicit go version, even though it needs to be updated from time to time. And then sync this with the version in go.mod, which currently is at 1.13...

For the cothority, we tested the two latest go versions with a matrix, which would be 1.21 and 1.22 here. And put the lower version in go.mod, so it's a little bit more backwards-compatible.

[pierluca]

Done