Revert "[Breaking change] Align return types across assertions / assu…

…mptions / soft assertions and do not use Abstract Asserts"

This reverts commit cfbb302 (#2173) due to the heavy impact on binary
compatibility and to continue allowing third-party libraries returning
their own assertion classes with overridden `assertThat` methods.

src/main/java/org/assertj/core/api/AssertionsForInterfaceTypes.java

@@ -92,7 +92,7 @@ protected AssertionsForInterfaceTypes() {}
    * @param actual the actual value.
    * @return the created assertion object.
    */
-  public static CharSequenceAssert assertThat(CharSequence actual) {
+  public static AbstractCharSequenceAssert<?, ? extends CharSequence> assertThat(CharSequence actual) {
     return new CharSequenceAssert(actual);
   }
 
@@ -296,7 +296,7 @@ ClassBasedNavigableListAssert<?, ACTUAL, ELEMENT, ELEMENT_ASSERT> assertThat(Lis
    * @param actual the path to test
    * @return the created assertion object
    */
-  public static PathAssert assertThat(Path actual) {
+  public static AbstractPathAssert<?> assertThat(Path actual) {
     return new PathAssert(actual);
   }
 
@@ -323,7 +323,7 @@ public static <K, V> MapAssert<K, V> assertThat(Map<K, V> actual) {
    * @param actual the actual value.
    * @return the created assertion object.
    */
-  public static <T extends Comparable<? super T>> GenericComparableAssert<T> assertThat(T actual) {
+  public static <T extends Comparable<? super T>> AbstractComparableAssert<?, T> assertThat(T actual) {
     return new GenericComparableAssert<>(actual);
   }