Fix markstale.ps1 Digests parameter type from string to string[] (#601)

## Problem

The `MarkOldImagesStale` job in the "Build new images" workflow is
failing ([run
#695](https://github.com/microsoft/nav-docker/actions/runs/24763865306/job/72462802665)).

The `$Digests` parameter in `markstale.ps1` is declared as `[string]`,
but both callers pass an array:

- `BuildMissingImages.yaml`: `$digests = $env:digestsJson |
ConvertFrom-Json` (produces `string[]`)
- `MarkStale.yaml`: `$digests = "$env:digests".Split(",").Trim() |
Select-Object -Unique` (produces `string[]`)

PowerShell coerces the array into a single space-joined string (e.g.
`"sha256:abc sha256:def"`), so the `foreach` loop iterates once with a
malformed digest, causing `oras attach` to fail.

## Fix

Change the parameter type from `[string]` to `[string[]]` so the array
is preserved and each digest is processed individually.

Co-authored-by: aholstrup1 <aholstrup1@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aholstrup1

build/markstale.ps1

@@ -1,6 +1,6 @@
 param(
     [Parameter(Mandatory = $true)]
-    [string]$Digests, # $digests = $env:digestsJson | ConvertFrom-Json
+    [string[]]$Digests, # $digests = $env:digestsJson | ConvertFrom-Json
     [Parameter(Mandatory = $false)]
     [string]$PushRegistry = "mcrbusinesscentral.azurecr.io"
 )