Upgrade build and test toolchain to current baseline

[lipkau]

Problem

The repository pins outdated build/test dependencies (including Pester 4 and legacy PlatyPS), making it harder to share tooling patterns with JiraPS and increasing maintenance overhead.

Proposal

Upgrade the build/test toolchain to align with current AtlassianPS standards:

• Pester 5 migration
• Microsoft.PowerShell.PlatyPS migration
• updated PSScriptAnalyzer and InvokeBuild versions
• modernized setup/build scripts that preserve cross-platform behavior

Acceptance criteria

• Tools/build.requirements.psd1 reflects updated dependency versions
• Test execution works under Pester 5 across supported environments
• Help generation/build tasks run successfully with modern PlatyPS
• Static analysis runs cleanly with updated PSScriptAnalyzer rules
• CI pipeline passes using the upgraded toolchain