Upgrade to Pester 5.x

[hmmwhatsthisdo]

Context

Is your feature request related to a problem? Please describe.

Using an older version of Pester makes it more difficult for contributors to iterate. Currently, we depend on Pester v4.4 - Pester v4 tests are not forwards-compatible with Pester 5.

Description

Describe the solution you'd like

We should upgrade to Pester 5.

Additional Information

Using a more-recent version of Pester enables tooling like the Pester VS Code extension, which provides contributors with significant QoL improvements (e.g. continuous test runs).