Check we're on the right branch before tagging, and on the right tag before uploading

* Abort if we're on the wrong branch
* Check we have the right tag checked out
* Clarify that `publish` only releases to GitHub

Not enforced in config yet. One day.

Poetry now manages the project version in pyproject.toml.

The release script used to incorrectly write `New synapse release 1.57.0~rc1.`
instead of `New synapse release 1.57.0rc1.`

As we want to include the previous version in the "No new changes..."
string.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* rename major/minor into the right semver terminology minor/patch (since this was something that got me very confused the first couple of times I've used the script)
* name the release branch based on the new version, not the previous one

With the prior format, 1.33.0 / 1.33.1 / 1.33.2 got separate branches:

    release-v1.33.0
    release-v1.33.1
    release-v1.33.2

Under the new model, all three would share a common branch:

    release-v1.33

As before, RCs and actual releases exist as tags on these branches.

This better reflects our support model, e.g., that the "1.33" series had
a formal release followed by two patches / updates.

Signed-off-by: Dan Callahan <danc@element.io>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>