Change SemanticVersion's PreReleaseTag and BuildMetaData to a non-nullable type

To avoid warning CS8602: Dereference of a possibly null reference.

Author: AlexPykavy

src/GitVersion.Core/VersionCalculation/SemanticVersioning/SemanticVersion.cs

@@ -21,8 +21,8 @@ public class SemanticVersion : IFormattable, IComparable<SemanticVersion>, IEqua
     public long Major;
     public long Minor;
     public long Patch;
-    public SemanticVersionPreReleaseTag? PreReleaseTag;
-    public SemanticVersionBuildMetaData? BuildMetaData;
+    public SemanticVersionPreReleaseTag PreReleaseTag;
+    public SemanticVersionBuildMetaData BuildMetaData;
 
     public bool HasPreReleaseTagWithLabel => PreReleaseTag?.HasTag() == true;