Remove outdated reference to `EnvironmentTestUtils`

Documentation still refers to the now deprecated `EnvironmentTestUtils`
class instead of `TestPropertyValues` for setting environment values in
tests.

This commit replaces it with `TestPropertyValues` and provides an
equivalent code example.

Closes gh-12711

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -7160,14 +7160,14 @@ which auto-configures one for you.
 
 
 [[boot-features-environment-test-utilities]]
-==== EnvironmentTestUtils
-`EnvironmentTestUtils` lets you quickly add properties to a
+==== TestPropertyValues
+`TestPropertyValues` lets you quickly add properties to a
 `ConfigurableEnvironment` or `ConfigurableApplicationContext`. You can call it with
 `key=value` strings, as follows:
 
 [source,java,indent=0]
 ----
-	EnvironmentTestUtils.addEnvironment(env, "org=Spring", "name=Boot");
+	TestPropertyValues.of("org=Spring", "name=Boot").applyTo(env);
 ----