feat: Change party identifier separator to single colon (#746)

<!--- Provide a general summary of your changes in the Title above -->

## Description
Change party identifier separator to single colon

<!--- Describe your changes in detail -->

## Related Issue(s)

- #700 

## Verification

- [x] **Your** code builds clean without any errors or warnings
- [x] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)

Co-authored-by: Ole Jørgen Skogstad <skogstad@softis.net>

src/Digdir.Domain.Dialogporten.Domain/Parties/Abstractions/PartyIdentifier.cs

@@ -7,15 +7,15 @@ public static class PartyIdentifier
 {
     private delegate bool TryParseDelegate(ReadOnlySpan<char> value, [NotNullWhen(true)] out IPartyIdentifier? identifier);
     private static readonly Dictionary<string, TryParseDelegate> TryParseByPrefix = CreateTryParseByPrefix();
-    public const string Separator = "::";
+    public const string Separator = ":";
 
     public static string Prefix(this IPartyIdentifier identifier)
         => identifier.FullId[..(identifier.FullId.IndexOf(identifier.Id, StringComparison.Ordinal) - Separator.Length)];
 
     public static bool TryParse(ReadOnlySpan<char> value, [NotNullWhen(true)] out IPartyIdentifier? identifier)
     {
         identifier = null;
-        var separatorIndex = value.IndexOf(Separator);
+        var separatorIndex = value.LastIndexOf(Separator);
         if (separatorIndex == -1)
         {
             return false;
@@ -28,7 +28,7 @@ public static bool TryParse(ReadOnlySpan<char> value, [NotNullWhen(true)] out IP
 
     internal static ReadOnlySpan<char> GetIdPart(ReadOnlySpan<char> value)
     {
-        var separatorIndex = value.IndexOf(Separator);
+        var separatorIndex = value.LastIndexOf(Separator);
         return separatorIndex == -1
             ? value
             : value[(separatorIndex + Separator.Length)..];