[docs] Old link to actions/virtual-environments#available-environments in README

[jtomkiew-mng]

Link https://github.com/actions/virtual-environments#available-environments from:

powershell/README.md

Line 76 in 08700f2

To use the latest Az module version, specify 'latest'. You can find the list of Az module versions that can be given as azPSVersion from this [table](https://github.com/actions/virtual-environments#available-environments).

should be updated to: https://github.com/actions/runner-images#available-images.

(Repo redirect works on GitHub level, but the HTML content ID in the linked README was changed from available-environments to available-images)