update readme for min gradle version

[cherylking]

Fixes #857

It has been reported that running our plugin version 3.7 with versions of Gradle < 7.6 runs into the exception described in the referenced issue. This is a known problem in Gradle related to the ASM version being used. I am updating the README to indicate a minimum version of 7.6 is required, and will update the release notes also.

Running a project locally with 7.5, I saw the following stack trace:

Caused by: java.io.IOException: Failed to process the entry 'META-INF/versions/19/com/fasterxml/jackson/core/io/doubleparser/FastDoubleSwar.class' from '/Users/cherylking/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.15.2/jackson-core-2.15.2.jar'
        at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.lambda$visitEntries$1(InstrumentingClasspathFileTransformer.java:161)
        at org.gradle.internal.classpath.ClasspathWalker.visitJarContents(ClasspathWalker.java:91)
        at org.gradle.internal.classpath.ClasspathWalker.visit(ClasspathWalker.java:55)
        at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.visitEntries(InstrumentingClasspathFileTransformer.java:148)
        at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.lambda$instrument$0(InstrumentingClasspathFileTransformer.java:139)
        at org.gradle.internal.classpath.ClasspathBuilder.buildJar(ClasspathBuilder.java:66)
        at org.gradle.internal.classpath.ClasspathBuilder.jar(ClasspathBuilder.java:53)
        ... 9 more
Caused by: java.lang.IllegalArgumentException: Unsupported class file major version 63
        at org.objectweb.asm.ClassReader.<init>(ClassReader.java:199)
        at org.objectweb.asm.ClassReader.<init>(ClassReader.java:180)
        at org.objectweb.asm.ClassReader.<init>(ClassReader.java:166)
        at org.gradle.internal.classpath.InstrumentingClasspathFileTransformer.lambda$visitEntries$1(InstrumentingClasspathFileTransformer.java:151)
        ... 15 more