fix: Get-Culture -Name parameter not available in PS5.1

[HeyItsGilbert]

What to build

The test at tests/Get-PlasterManifestPathForCulture.Tests.ps1:22 calls Get-Culture -name "en-US", which fails on PS5.1 because the -Name parameter was added in PS6. The other two tests in the same file already use New-Object System.Globalization.CultureInfo("en-US"). Replace the one offending call to match that pattern.

Acceptance criteria

• Get-PlasterManifestPathForCulture — "returns the manifest for the specified culture" passes on PS5.1
• No change in behaviour on PS7

Blocked by

None - can start immediately