Let's change the default branch to `main`

It's overdue to change the default branch to main.

@nagyv can you change that in the settings?

I can spot a reference to master here:

https://github.com/gitlabhq/terraform-provider-gitlab/blob/8cd2568e63320d127d215b1662a926d58186b8a2/.github/workflows/pr-check-conflicts.yml#L10

However, main is already included in that list, thus it shouldn't be an immediate problem. However, master can be removed :)

... and in the README when referencing the badges.

There is other master references in the test examples, which can also be changed independently.