Add Proguard 7.0.0 to third_party.

Change-Id: I9d8b6fd6cc9e374b1cbbaf7e43bd83f22978c1b7
diff --git a/build.gradle b/build.gradle
index 4428f58..693cdba 100644
--- a/build.gradle
+++ b/build.gradle
@@ -334,6 +334,7 @@
                 "openjdk/jdk-11-test",
                 "proguard/proguard5.2.1",
                 "proguard/proguard6.0.1",
+                "proguard/proguard-7.0.0",
                 "r8",
                 "r8-releases/2.0.74",
                 "r8mappings",