📖 [Docs]: Document the Pester major-version lock in the test pipeline (#362)

The pipeline README now documents that module test files lock Pester to
the **6.x** major and the Invoke-Pester step installs a matching
version, so contributors understand how the test framework is
constrained.

## Changed: README documents the Pester major-version lock

The "Test module" section now states the `#Requires` form test files use
and notes that minor and patch updates flow in automatically while a new
major stays a deliberate, reviewed change.

## Technical Details

- Updates the "Test module" section of `README.md`.
- Part of the dependency-management epic #356.

Author: MariusStorhaug

README.md

@@ -176,6 +176,7 @@ The [PSModule - SourceCode tests](./scripts/tests/SourceCode/PSModule/PSModule.T
 
 [workflow](./.github/workflows/Test-ModuleLocal.yml)
 - Imports and tests the module in parallel (matrix) using Pester tests from the module repository.
+- Module test files declare a Pester **6.x** requirement via `#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }` — a convention module authors add to each `*.Tests.ps1`, not something this pipeline injects. The [Invoke-Pester](https://github.com/PSModule/Invoke-Pester) action installs a matching `6.x`, so minor and patch updates flow in automatically while a new major stays a deliberate, reviewed change.
 - Supports setup and teardown scripts executed via separate dedicated jobs:
   - `BeforeAll`: Runs once before all test matrix jobs to set up the test environment (e.g., deploy infrastructure, download test data).
   - `AfterAll`: Runs once after all test matrix jobs complete to clean up the test environment (e.g., remove test resources, clean up databases).