Remove temporary type parameters.

This will break most users who static import both `Truth.assertThat` and `Truth8.assertThat`. The fix is usually as simple as replacing every reference to `Truth8` with a reference to `Truth`. But we'll post some additional migration information as part of the release notes, as we've already done for [1.3.0](https://github.com/google/truth/releases/tag/v1.3.0) and [1.4.0](https://github.com/google/truth/releases/tag/v1.4.0).

(The type parameters existed to avoid that static import conflict. However, the type parameters also _cause other static import conflicts_, so we don't want them in place in the long term.)

This is one of the remaining loose ends of #746.

RELNOTES=Removed temporary type parameters from `Truth.assertThat(Stream)` and `Truth.assertThat(Optional)`. This can create build errors, which you can fix by replacing all your references to `Truth8` with references to `Truth`.
PiperOrigin-RevId: 604754613

Author: cpovirk

core/src/main/java/com/google/common/truth/Truth.java

@@ -265,7 +265,7 @@ public static TableSubject assertThat(@Nullable Table<?, ?, ?> actual) {
     "Java7ApiChecker", // no more dangerous than wherever the user got the Optional
     "NullableOptional", // Truth always accepts nulls, no matter the type
   })
-  public static <T> OptionalSubject assertThat(@Nullable Optional<T> actual) {
+  public static OptionalSubject assertThat(@Nullable Optional<?> actual) {
     return assert_().that(actual);
   }
 
@@ -297,7 +297,7 @@ public static OptionalDoubleSubject assertThat(@Nullable OptionalDouble actual)
    * @since 1.4.0 (present in {@link Truth8} since before 1.0)
    */
   @SuppressWarnings("Java7ApiChecker") // no more dangerous than wherever the user got the Stream
-  public static <T extends @Nullable Object> StreamSubject assertThat(@Nullable Stream<T> actual) {
+  public static StreamSubject assertThat(@Nullable Stream<?> actual) {
     return assert_().that(actual);
   }