Document that truth-java8-extension is obsolete.

Also, remove its `jspecify` dep. (But leave behind its Truth dep so that anyone who depends only on `truth-java8-extension` for `Truth`+`Truth8` still gets them.)

I could probably further simplify in the pom's build config, but there's no compelling need.

RELNOTES=n/a
PiperOrigin-RevId: 650668773

extensions/java8/pom.xml

@@ -10,19 +10,15 @@
     <version>HEAD-SNAPSHOT</version>
   </parent>
   <artifactId>truth-java8-extension</artifactId>
-  <name>Truth Extension for Java8</name>
+  <name>Obsolete Truth Extension for Java8</name>
   <description>
-    An extension for the Truth test assertion framework supporting Java8 types and structures
+    Obsolete, empty artifact that merely pulls in the main `truth` artifact: Assertions for Java 8 types are now part of that main artifact.
   </description>
   <dependencies>
     <dependency>
       <groupId>com.google.truth</groupId>
       <artifactId>truth</artifactId>
     </dependency>
-    <dependency>
-      <groupId>org.jspecify</groupId>
-      <artifactId>jspecify</artifactId>
-    </dependency>
   </dependencies>
   <build>
     <resources>