[Feature]: Support of multi mainline branches in the Mainline version strategy

[HHobeck]

Prerequisites

• I have written a descriptive issue title
• I have searched issues to ensure it has not already been reported

GitVersion package

GitVersion.Tool

GitVersion version

6.0.0-rc.1

Operating system

N/A

What are you seeing?

The support of multi mainline branches (where the is-main-branch property is set to true) needs to be implemented.

GIVEN a Mainline version strategy with the GitFlow/v1 workflow
WHEN a main -> support/1.x branch is used
THEN a NotImplementedException in MainlineVersionStrategy.cs is thrown

What is expected?

A semantic version needs to be calculated

Steps to Reproduce

Create a commit on main
branch to support/1.x
create a commit on support/1.x
run GitVersion

RepositoryFixture Test

No response

Output log or link to your CI build (if appropriate).

No response