⚙️ [Maintenance]: Lock Pester test dependency to the 6.x major version (#25)

Modules scaffolded from this template now inherit a Pester **6.x**
major-version requirement in their test file, so every new module starts
aligned with the rest of the ecosystem.

## Changed: the module test template requires Pester 6.x

`tests/PSModuleTest.Tests.ps1` now starts with:

```powershell
#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }
```

Any Pester `6.x` satisfies it; adopting a new major stays a deliberate
change.

## Technical Details

- Adds a `#Requires -Modules` statement to the template's test file.
Module-identity (`GUID`) pinning is intentionally omitted — a separate
supply-chain control, not part of the lock-to-major risk appetite.
- The install side is locked to the same major in
PSModule/Invoke-Pester#70. Part of the dependency-management epic
PSModule/Process-PSModule#356.

Author: MariusStorhaug

tests/PSModuleTest.Tests.ps1

@@ -1,4 +1,6 @@
-[Diagnostics.CodeAnalysis.SuppressMessageAttribute(
+#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }
+
+[Diagnostics.CodeAnalysis.SuppressMessageAttribute(
     'PSReviewUnusedParameter', '',
     Justification = 'Required for Pester tests'
 )]