Fix release process

[cprice404]

We need to clean up the release process, it has some issues right now.

• Set up a release branch and a manual release action like we have for some of the SDKs
• There is an action in the github cd workflow that tries to update the cargo.toml file whenever a pr is merged. it does this on a branch, but then nothing seems to happen after that, so we have accumulated a ton of those branches. We should remove that action from the "push" workflow
• clean up the old branches from that job
• Update the release workflow so that it does properly update the cargo.toml file
• Verify that --version shows the right value for a released version
• general audit of the rest of the stuff in the github workflows to make sure it looks reasonable.
• there is a .semrel dir committed to this repo, which I don't believe is actually used for anything; double-check and remove it.

[cprice404]

Work for this ticket is probably relevant for #181 as well.

[poppoerika]