`New-SampleModule`: call out `MainGitBranch` default (`main`) as a behavior change

[raandree]

Problem description
PR #560 introduced -MainGitBranch on New-SampleModule defaulting to main. Historically, projects scaffolded from the CustomModule Plaster template defaulted to master in several places. The new default is sensible but is a user-visible default-behavior change, and the changelog only mentions it under ### Added.

Verbose logs
N/A

How to reproduce

1. Scaffold a project with New-SampleModule -ModuleType CustomModule ... before and after PR CustomModule fixes and Copilot repo guidance #560.
2. Diff Git-branch references in generated files (azure-pipelines.yml, README badges, GitVersion config, etc.).

Expected behavior

• CHANGELOG.md has a ### Changed entry stating the default branch is now main and that users relying on master must pass -MainGitBranch master.
• All template files that reference a default branch consume MainGitBranch rather than hard-coding master or main.

Current behavior
The default change is only documented as an ### Added entry; template files have not been audited for hard-coded branch names in this PR.

Suggested solution

• Add a ### Changed entry for the default-branch change.
• Audit Sampler/Templates/Sampler/** for any remaining hard-coded master/main references and route them through ${PLASTER_PARAM_MainGitBranch}.

Operating system the target node is running
N/A

PowerShell version and build the target node is running
N/A

Module version used
Sampler Unreleased / post-#560.