Raise the minimum supported version of Gradle 8 to 8.4

[wilkinsona]

Upgrading to Jackson 2.18 (#42480) reveals an incompatibility with Gradle 8.3:

Caused by: java.io.IOException: Failed to process the entry 'META-INF/versions/22/com/fasterxml/jackson/core/internal/shaded/fdp/v2_18_0/FastDoubleSwar.class' from '/Users/awilkinson/.gradle/caches/modules-2/files-2.1/com.fasterxml.jackson.core/jackson-core/2.18.0/65e8ead7de5d8f7a53e296c363bea3182f21f925/jackson-core-2.18.0.jar'
	at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.lambda$visitEntries$1(InstrumentingClasspathFileTransformer.java:222)
	at org.gradle.internal.classpath.ClasspathWalker.visitJarContents(ClasspathWalker.java:91)
	at org.gradle.internal.classpath.ClasspathWalker.visit(ClasspathWalker.java:55)
	at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.visitEntries(InstrumentingClasspathFileTransformer.java:201)
	at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.lambda$instrument$0(InstrumentingClasspathFileTransformer.java:192)
	at org.gradle.internal.classpath.ClasspathBuilder.buildJar(ClasspathBuilder.java:68)
	at org.gradle.internal.classpath.ClasspathBuilder.jar(ClasspathBuilder.java:55)
	... 9 more
Caused by: java.lang.IllegalArgumentException: Unsupported class file major version 66
	at org.objectweb.asm.ClassReader.<init>(ClassReader.java:199)
	at org.objectweb.asm.ClassReader.<init>(ClassReader.java:180)
	at org.objectweb.asm.ClassReader.<init>(ClassReader.java:166)
	at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.lambda$visitEntries$1(InstrumentingClasspathFileTransformer.java:207)
	... 15 more

We can work around this with some complex build setup that downgrades Jackson to 2.17 when using Gradle 8.3. Alternatively, we can avoid that by raising the minimum supported version of Gradle to 8.4. Gradle 8.4 was released in October 2023 so it will have been available for over a year by the time Boot 3.4 GAs so I think we should raise the minimum supported Gradle version.