Revert to Java 11, tests currently fail under Java 21

The version of the mocking library Mockito that we're currently using does
not support Java 21, and so the tests currently fail under Java 21 with
`java.lang.NoClassDefFoundError`

```
[info] com.gu.facia.api.contentapi.ContentApiTest *** ABORTED ***
[info]   java.lang.NoClassDefFoundError: Could not initialize class org.mockito.internal.creation.cglib.ClassImposterizer$3
[info]   at org.mockito.internal.creation.cglib.ClassImposterizer.createProxyClass(ClassImposterizer.java:95)
[info]   at org.mockito.internal.creation.cglib.ClassImposterizer.imposterise(ClassImposterizer.java:57)
[info]   at org.mockito.internal.creation.cglib.ClassImposterizer.imposterise(ClassImposterizer.java:49)
[info]   at org.mockito.internal.creation.cglib.CglibMockMaker.createMock(CglibMockMaker.java:24)
[info]   at org.mockito.internal.util.MockUtil.createMock(MockUtil.java:33)
[info]   at org.mockito.internal.MockitoCore.mock(MockitoCore.java:59)
[info]   at org.mockito.Mockito.mock(Mockito.java:1285)
[info]   at org.mockito.Mockito.mock(Mockito.java:1163)
[info]   at org.scalatestplus.mockito.MockitoSugar.mock(MockitoSugar.scala:73)
[info]   at org.scalatestplus.mockito.MockitoSugar.mock$(MockitoSugar.scala:72)
[info]   ...
[info]   Cause: java.lang.ExceptionInInitializerError: Exception java.lang.ExceptionInInitializerError [in thread "pool-7-thread-11"]
[info]   at org.mockito.cglib.core.KeyFactory$Generator.generateClass(KeyFactory.java:167)
[info]   at org.mockito.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
[info]   at org.mockito.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:217)
[info]   at org.mockito.cglib.core.KeyFactory$Generator.create(KeyFactory.java:145)
[info]   at org.mockito.cglib.core.KeyFactory.create(KeyFactory.java:117)
[info]   at org.mockito.cglib.core.KeyFactory.create(KeyFactory.java:109)
[info]   at org.mockito.cglib.core.KeyFactory.create(KeyFactory.java:105)
[info]   at org.mockito.cglib.proxy.Enhancer.<clinit>(Enhancer.java:70)
[info]   at org.mockito.internal.creation.cglib.ClassImposterizer.createProxyClass(ClassImposterizer.java:95)
[info]   at org.mockito.internal.creation.cglib.ClassImposterizer.imposterise(ClassImposterizer.java:57)
[info]   ...
```

.tool-versions

@@ -1 +1 @@
-java corretto-21.0.3.9.1
+java corretto-11.0.23.9.1