⚙️ [Maintenance]: Adopt Process-PSModule v6.1.4 and require Pester 6.x in tests (#67)

This module now runs on the latest Process-PSModule automation (v6.1.4)
and pins its test suite to Pester 6, so continuous integration uses the
current build, test, and publish pipeline and the tests fail fast if
Pester 6 is ever missing.

## Changed: CI runs on Process-PSModule v6.1.4

The reusable workflow pin moves to v6.1.4 from the module's previously
pinned v5 release, adopting the current pipeline, including the Pester 6
test runner and the consolidated `TestData` secret model. No caller
changes are required — the existing `secrets` wiring keeps passing
everything the pipeline needs.

## Changed: Tests require Pester 6.x

Every test file now declares the Pester 6 requirement, so the suite
always resolves Pester 6 and fails with a clear message if only an older
Pester is available, instead of silently running on it.

## Technical Details

- `.github/workflows/Process-PSModule.yml`: pin bumped to
`workflow.yml@da180bac16b13bfbcdf08b2e4e221b5b49e5ff28 # v6.1.4`.
- `tests/*.Tests.ps1`: added `#Requires -Modules @{ ModuleName =
'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }`.
- Classic `Should` assertions are left unchanged. Pester 6 is
backward-compatible, so converting to the new `Should-*` syntax is
intentionally out of scope for this maintenance change.
- Supersedes any stale Dependabot bump of the Process-PSModule pin.

Author: MariusStorhaug

tests/Sodium.Tests.ps1

@@ -1,4 +1,6 @@
-Describe 'Sodium' {
+#Requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '6.0.0'; MaximumVersion = '6.*' }
+
+Describe 'Sodium' {
     Context 'SealedBox - Encryption and Decryption' {
         It 'Encrypts and decrypts a message correctly using valid keys' {
             $keyPair = New-SodiumKeyPair