Refactor AssertJ assertions into more idiomatic ones

This commit refactors some AssertJ assertions into more idiomatic and
readable ones. Using the dedicated assertion instead of a generic one
will produce more meaningful error messages. 

For instance, consider collection size:
```
// expected: 5 but was: 2
assertThat(collection.size()).equals(5);
// Expected size: 5 but was: 2 in: [1, 2]
assertThat(collection).hasSize(5);
```

Closes gh-30104

spring-aspects/src/test/java/org/springframework/cache/aspectj/AbstractCacheAnnotationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -543,7 +543,7 @@ public abstract class AbstractCacheAnnotationTests {
 		Object r1 = service.multiConditionalCacheAndEvict(key);
 		Object r3 = service.multiConditionalCacheAndEvict(key);
 
-		assertThat(!r1.equals(r3)).isTrue();
+		assertThat(r1.equals(r3)).isFalse();
 		assertThat(primary.get(key)).isNull();
 
 		Object key2 = 3;

spring-aspects/src/test/java/org/springframework/cache/aspectj/AspectJEnableCachingIsolatedTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -92,7 +92,7 @@ public class AspectJEnableCachingIsolatedTests {
 			load(MultiCacheManagerConfig.class);
 		}
 		catch (IllegalStateException ex) {
-			assertThat(ex.getMessage().contains("bean of type CacheManager")).isTrue();
+			assertThat(ex.getMessage()).contains("bean of type CacheManager");
 		}
 	}
 
@@ -107,7 +107,7 @@ public class AspectJEnableCachingIsolatedTests {
 			load(MultiCacheManagerConfigurer.class, EnableCachingConfig.class);
 		}
 		catch (IllegalStateException ex) {
-			assertThat(ex.getMessage().contains("implementations of CachingConfigurer")).isTrue();
+			assertThat(ex.getMessage()).contains("implementations of CachingConfigurer");
 		}
 	}
 
@@ -117,7 +117,7 @@ public class AspectJEnableCachingIsolatedTests {
 			load(EmptyConfig.class);
 		}
 		catch (IllegalStateException ex) {
-			assertThat(ex.getMessage().contains("no bean of type CacheManager")).isTrue();
+			assertThat(ex.getMessage()).contains("no bean of type CacheManager");
 		}
 	}