refactor(GitVersion.Configuration): add BeforeString to avoid serializing Nullable<DateTimeOffset>

Nullable types seem to ignore the types' built-in JSON converters
i.e. DateTimeOffset <=> string, but DateTimeOffset? <=> object or null

Author: BinToss

src/GitVersion.Configuration/IgnoreConfiguration.cs

@@ -4,10 +4,18 @@ namespace GitVersion.Configuration;
 
 internal record IgnoreConfiguration : IIgnoreConfiguration
 {
+    [JsonIgnore]
+    public DateTimeOffset? Before { get; init; }
+
     [JsonPropertyName("commits-before")]
     [JsonPropertyDescription("Commits before this date will be ignored. Format: yyyy-MM-ddTHH:mm:ss.")]
     [JsonPropertyFormat(Format.DateTime)]
-    public DateTimeOffset? Before { get; init; }
+    public string? BeforeString
+    {
+        get => Before?.ToString("yyyy-MM-ddTHH:mm:ssZ");
+        // if TryParse returns false, Before = null and the parse error is suppressed. How should the user be informed of parse errors?
+        init => Before = value is null ? null : DateTimeOffset.TryParse(value, out DateTimeOffset dto) ? dto : null;
+    }
 
     [JsonIgnore]
     IReadOnlyCollection<string> IIgnoreConfiguration.Shas => Shas;