Use online checksums for PowerShell in .NET SDK Dockerfiles

This is part of #6548. Instead of using hard-coded checksums, they should be downloaded from the build/release storage account. This makes the Dockerfiles more flexible and simplifies this repo's maintenance, since we will no longer need to calculate or keep track of checksums.

For a given PowerShell release, checksums are available at: `https://powershellinfraartifacts-gkhedzdeaghdezhr.z01.azurefd.net/tool/$powershell_version/SHA512SUMS`. For example: https://powershellinfraartifacts-gkhedzdeaghdezhr.z01.azurefd.net/tool/7.4.11/SHA512SUMS

Example:
https://github.com/dotnet/dotnet-docker/blob/157267f8403b1985704cb6810a5413fc7e3a1bc2/src/runtime/10.0/alpine3.22/amd64/Dockerfile#L7-L16