Automate version constant update in built-in connectors

Built-in connectors need to know their correct version at compile time without tricks like fetching the version from the git tag, because they are imported as libraries in Conduit. Right now we store the version in a constant (e.g. [here](https://github.com/ConduitIO/conduit-connector-generator/blob/249843c8491d0c26e1bae683bc8fe65f732fe7d8/spec.go#L24)) but that needs to be updated manually and we will forget it. We should automate the updating of this constant through a CI action.

The goal is to ensure that a release (git tag) always sits on a commit in which the version constant matches the release tag. Right after a release the version should _ideally_ be updated to the next minor version with a development suffix (e.g. `v0.4.0-develop`). This is just one way to model the workflow, open to suggestions!