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

Change-Id: I6558f0b4db0f9a192c18bbe45e2eaf10595bc666
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Commit.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Commit.java
index 209827d..f14e758 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Commit.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Commit.java
@@ -4,9 +4,10 @@
 
 package eu.mulk.jgvariant.ostree;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 import eu.mulk.jgvariant.core.Decoder;
 import java.nio.ByteOrder;
-import java.nio.charset.StandardCharsets;
 import java.util.List;
 
 /**
@@ -48,8 +49,7 @@
   public record RelatedObject(String ref, Checksum commitChecksum) {
 
     private static final Decoder<RelatedObject> DECODER =
-        Decoder.ofStructure(
-            RelatedObject.class, Decoder.ofString(StandardCharsets.UTF_8), Checksum.decoder());
+        Decoder.ofStructure(RelatedObject.class, Decoder.ofString(UTF_8), Checksum.decoder());
 
     public static Decoder<RelatedObject> decoder() {
       return DECODER;
@@ -62,8 +62,8 @@
           Metadata.decoder(),
           Checksum.decoder(),
           Decoder.ofArray(RelatedObject.decoder()),
-          Decoder.ofString(StandardCharsets.UTF_8),
-          Decoder.ofString(StandardCharsets.UTF_8),
+          Decoder.ofString(UTF_8),
+          Decoder.ofString(UTF_8),
           Decoder.ofLong().withByteOrder(ByteOrder.BIG_ENDIAN),
           Checksum.decoder(),
           Checksum.decoder());