Use transcripts from mehari rather than dotty

[holtgrewe]

Is your feature request related to a problem? Please describe.
We want to use rfl in varfish-server as well. However, we do not run dotty there (as it is not needed). Mehari now offers the same functionality (varfish-org/mehari#339). Also rfl has been adjusted accordingly already (bihealth/reev-frontend-lib#115).

Describe the solution you'd like
Integrate the rfl release v0.6.1 and prefer Mehari over dotty for transcript information.

Describe alternatives you've considered
N/A

Additional context

• Add server endpoint for serving a gene's transcripts varfish-org/mehari#339
• Use transcript info from mehari server reev-frontend-lib#115