Convert some interfaces to final classes

Originally did this to make things easier to write, but after a few
months I've come to realize that yeah keeping them as utility classes is
just better.

Author: Jonathing

src/main/java/net/minecraftforge/gitver/internal/GitChangelog.java

@@ -22,7 +22,9 @@
  * Typically used by GitVersion in
  * {@link net.minecraftforge.gitver.api.GitVersion#generateChangelog(String, String, boolean)}.
  */
-interface GitChangelog {
+final class GitChangelog {
+    private GitChangelog() { }
+
     /**
      * Generates a changelog string that can be written to a file from a given git directory and repository url.
      * <p>

src/main/java/net/minecraftforge/gitver/internal/GitUtils.java

@@ -39,7 +39,9 @@
  * This is used heavily by, and in conjunction with, GitVersion. It holds the majority of operations done on the Git
  * repository.
  */
-interface GitUtils {
+final class GitUtils {
+    private GitUtils() { }
+
     /**
      * Gets the relative path of a file from a root directory. Uses NIO's {@link Path} to guarantee cross-platform
      * compatibility and reproducible path strings.

src/main/java/net/minecraftforge/gitver/internal/Util.java

@@ -22,7 +22,7 @@
 import java.util.function.Supplier;
 import java.util.function.UnaryOperator;
 
-public final class Util {
+final class Util {
     /**
      * Hacky method to throw a checked exception without declaring it.
      *