Use Github Actions for CI

[mlarraz]

Recent commits have not been building on Travis:

Travis' support for open source is significantly limited now, so switching to Github is a good alternative.

See my build here https://github.com/mlarraz/standalone-migrations/runs/6198591168

This makes it clear that support for Rails 7 is broken, as documented in https://github.com/thuss/standalone-migrations/issues/167 https://github.com/thuss/standalone-migrations/issues/169 and https://github.com/thuss/standalone-migrations/issues/170.

Hoping that this will highlight the issue and lead to my fix in https://github.com/thuss/standalone-migrations/pull/171 getting merged.