Pin Pester dependency to v4

[vexx32]

PR Summary

Pins the Pester version required for the module to 4.10.1 since RequiredModules doesn't appear to support a MaximumVersion, and also pins the build pipeline to maximum version of 4.99.99 for Pester.

Context

Currently users can install Pester v5 and PSKoans will be trying to utilise that version on import; this will cause things to break.

This should force PowerShellGet to retrieve the right version from the gallery when installing the new PSKoans version and avoid that breakage until I can update this module to work with v5 of Pester.

Changes

• Set RequiredVersion in the manifest for Pester
• Set -MaximumVersion in the CI pipeline for Pester

Checklist

• Pull Request has a meaningful title.
• Summarised changes.
• Pull Request is ready to merge & is not WIP.
• Added tests / only testable interactively.
  • Make sure you add a new test if old tests do not effectively test the code changed.
• Added documentation / opened issue to track adding documentation at a later date.