Make 3.0.x branch default for all new development

Problem/Motivation

A 3.0.x branch has been created in this repo and an initial 3.0.0 release was created from that branch (with D11 compatibility).

Most existing Active, Needs Work, Needs Review and RTBC issues and MRs are still targeting 8.x-2.x.

Proposed resolution

Ideally the 3.0.x branch should be:
- Configured as the default branch for the code repository in GitLab
- Used as the default target for all new issues / MRs created for this module

It would also be advisable to update the version / MR target branch for existing issues that aren't truly 8.x-2.x specific to 3.0.x.

Issues that need to be addressed in both 3.0.x and 8.x-2.x should ideally be addressed in 3.0.x first and then backported to 8.x-2.x.