⚙️ [Maintenance]: Require Pester 6.x in test files (#16)

Pester tests in this module now require the Pester **6.x** major
version, so every contributor and CI run resolves the same major and a
new Pester major can't slip in and break the suite. The tests previously
declared no framework requirement and ran on whatever Pester happened to
be installed.

## Changed: tests are locked to the Pester 6.x major

Every `*.Tests.ps1` file now starts with a version-bounded requirement:

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

Any Pester `6.x` satisfies it, so minor and patch releases flow in
automatically while moving to a new major stays a deliberate, reviewed
change. No module source or behaviour changes.

## Technical Details

- Prepends a `#Requires -Modules` statement to each test file under
`tests/`; non-test files are untouched.
- `ModuleVersion = '6.0.0'` is the floor and `MaximumVersion = '6.*'`
the wildcard major ceiling. Module-identity (`GUID`) pinning is
intentionally omitted — it is 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 @@
-Describe 'Module' {
+#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }
+
+Describe 'Module' {
     It 'Function: Get-PSModuleTest' {
         Get-PSModuleTest -Name 'World' | Should -Be 'Hello, World!'
     }