Clean up Gemfile

[Splines]

• Pin gem versions instead of using the newest commit on the main branch
• Don't use >=, but ~> instead, which will not automatically install major version updates
• Clean up comments in the Gemfile and remove old packages that were commented out
• Remove SQLite dependency: as far as I can tell, we don't use SQLite anywhere, is that correct? We have been using PostgreSQL instead for a long time.

TODO

• [ ] Pin version for every single gem! (Is there a bundler command for this that we could use?)