Remove deps on truth-java8-extension.

Its classes [have been part of the core Truth artifact since 1.4.2](google/truth#746 (comment)).

Since we now depend on only one Truth artifact, I also inlined our `truth.version` property. Exception: I kept it in Guava, since we share it across multiple `pom.xml` files. (We _could_ accomplish the same thing by using `dependencyManagement` there, but [we have recently avoided it](https://github.com/google/guava/blob/257bc194f6e5bedd02115693f7416a84c3e4ba25/pom.xml#L107-L111), and I have some reservations around it, anyway, since it may override a newer version from a dependency. Fortunately, that is mostly academic now that we have Dependabot set up for almost all our projects.) The other thing about Guava is that we do _not_ have Dependabot set up for it. By keeping the version in a property, we allow our lesser Dependabot substitute, `versions-maven-plugin`, to bump it, as discussed on cl/554548816.

RELNOTES=n/a
PiperOrigin-RevId: 808216999

Author: cpovirk

guava-gwt/pom.xml

@@ -88,20 +88,6 @@
         </exclusion>
       </exclusions>
     </dependency>
-    <dependency>
-      <groupId>com.google.truth.extensions</groupId>
-      <artifactId>truth-java8-extension</artifactId>
-      <version>${truth.version}</version>
-      <classifier>gwt</classifier>
-      <scope>test</scope>
-      <exclusions>
-        <exclusion>
-          <!-- use the guava we're building. -->
-          <groupId>com.google.guava</groupId>
-          <artifactId>guava</artifactId>
-        </exclusion>
-      </exclusions>
-    </dependency>
   </dependencies>
   <repositories>
     <repository>

guava-tests/pom.xml

@@ -47,12 +47,6 @@
       <version>${truth.version}</version>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>com.google.truth.extensions</groupId>
-      <artifactId>truth-java8-extension</artifactId>
-      <version>${truth.version}</version>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>com.google.jimfs</groupId>
       <artifactId>jimfs</artifactId>