Bump Ruby from 2.6.6 to 2.7.6

[N0xFF]

Bundler can't install gems:

> bundle
Fetching gem metadata from https://rubygems.org/..........
activesupport-7.0.3.1 requires ruby version >= 2.7.0, which is incompatible with the current version, ruby 2.6.6p146

[jurre]

This ruby version is a bit misleading, the script runs using the Dockerfile, which bundles whatever version dependabot-core is on, which happens to be 2.7.6 currently, and will be 3.1 soon: https://github.com/dependabot/dependabot-core/pull/5447

[nikolamalesevic]

@jurre Can this version be removed altogether? Otherwise, it should be updated to 3.1.2, so it stops being misleading.