Add MaximumVersion to prevent loading latest installed Pester

[ajn142]

PR Summary

Sets maximum Pester version to 5.1.1
Resolves #462

Context

Submitting to address #462

Changes

• Added MaximumVersion property to Pester entry in PSKoans/PSKoans/PSKoans.psd1, set value to 5.1.1

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.

Not sure those last two are applicable.