TeamCity v6.0.0 feature branch testing

[modular-magician]

This PR enables nightly testing of the v6.0.0 feature branch

Schedule:

• On Saturday-Wednesday nightly tests will run off main branch of google and google-beta repos as ususal.
• On Thursdays the GA tests will run off the feature branch. Beta tests are off main as usual.
• On Fridays the Beta tests will run off the feature branch. GA tests are off main as usual.

What it looks like:

• There's a new project put inside the root "Google Cloud" project called "6.0.0 Major Release Testing"
• Inside there is a Google and Google Beta project, which each contain Nightly Test projects
• Those Nightly Test projects contain builds for all packages

Here is an example deployment: https://hashicorp.teamcity.com/project/TerraformProviders_SarahSTestProjectTpg?projectTab=overview&mode=builds

Screenshots of project layout :

Screenshots of the Beta cron triggers:

1. main branch

2. FEATURE-BRANCH-major-release-6.0.0 branch

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#11104