update towncrier command to new API

[m-vdb]

Proposed changes:

• Update release script to use towncrier API that changed in 21.9.0
  • towncrier changelog
  • Here is the blame of the poetry.lock file on main). The changes in rasa codebase are recent (I guess I was lucky to find the issue, the 3.0.8 changelog seems to be fine!)
  • They added a new --version option that prints towncrier version and exits, which makes our changelog generation fail silently. In addition they introduced a build command.
• Update changelog template to fix missing title issues

In order to test, you can run make generate-pending-changelog to see how this looks like now (no changes from before). To cleanup up your local directory after this, you can run make cleanup-generated-changelog

[m-vdb]

actually there is still an issue, it's not displaying the version name anymore 🤔

[m-vdb]

the template changed, I found the solution in this comment

[andreoid]

LGTM