Introduce TestSocketUtils as a replacement for SocketUtils

SocketUtils was officially deprecated in 5.3.16 (gh-28052) and removed
in 6.0 M3 (gh-28054); however, there is still need for a subset of this
functionality in integration tests for testing scenarios in which it is
not possible for the system under test to select its own random port
(or rely on the operating system to provide an ephemeral port).

This commit therefore introduces a scaled down version in the
spring-test module called TestSocketUtils which supports retrieval of a
single TCP port.

See gh-29132

spring-test/src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker

@@ -0,0 +1 @@
+mock-maker-inline