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

**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 #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.