Remove Checker Framework; add NullAway, Picnic Error Prone Support.

Change-Id: I6558f0b4db0f9a192c18bbe45e2eaf10595bc666
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DirTree.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DirTree.java
index dc38b1b..8ac9e67 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DirTree.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DirTree.java
@@ -4,8 +4,9 @@
 
 package eu.mulk.jgvariant.ostree;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 import eu.mulk.jgvariant.core.Decoder;
-import java.nio.charset.StandardCharsets;
 import java.util.List;
 
 /**
@@ -34,8 +35,7 @@
   public record File(String name, Checksum checksum) {
 
     private static final Decoder<File> DECODER =
-        Decoder.ofStructure(
-            File.class, Decoder.ofString(StandardCharsets.UTF_8), Checksum.decoder());
+        Decoder.ofStructure(File.class, Decoder.ofString(UTF_8), Checksum.decoder());
 
     /**
      * Acquires a {@link Decoder} for the enclosing type.
@@ -58,10 +58,7 @@
 
     private static final Decoder<Directory> DECODER =
         Decoder.ofStructure(
-            Directory.class,
-            Decoder.ofString(StandardCharsets.UTF_8),
-            Checksum.decoder(),
-            Checksum.decoder());
+            Directory.class, Decoder.ofString(UTF_8), Checksum.decoder(), Checksum.decoder());
 
     /**
      * Acquires a {@link Decoder} for the enclosing type.