Deprecate reuse-action

[carmenbianca]

The current implementation is not great because you can't fix a specific version of REUSE; you only ever get the latest release within a major version bracket.

We should output a deprecation warning before doing this.

[mxmehl]

How about changing the behaviour, and tie the action releases to the REUSE releases? Shouldn't be a big deal.

The action is used in >5000 repositories. Let's keep this user base in mind.