Upgrade to Gradle 8.7

[rwinch]

No description provided.

[rwinch]

After updating with the first commit, the build started reporting the following error:

> Task :artifactoryDeploy

FAILURE: Build failed with an exception.

* What went wrong:
A problem was found with the configuration of task ':spring-security-kerberos-client:signMavenJavaPublication' (type 'Sign').
  - In plugin 'org.gradle.plugins.signing.SigningPlugin$Inject' type 'org.gradle.plugins.signing.Sign' property 'generatorsByKey./home/runner/work/spring-security-kerberos/spring-security-kerberos/spring-security-kerberos-client/build/classes/java/main.toSign' file '/home/runner/work/spring-security-kerberos/spring-security-kerberos/spring-security-kerberos-client/build/classes/java/main' is not a file.
    
    Reason: Expected an input to be a file but it was a directory.
    
    Possible solutions:
      1. Use a file as an input.
      2. Declare the input as a directory instead.
    
    For more information, please refer to https://docs.gradle.org/8.7/userguide/validation_problems.html#unexpected_input_file_type in the Gradle documentation.

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Get more help at https://help.gradle.org./

BUILD FAILED in 1m 15s

This seems to be a duplicate of spring-projects/spring-statemachine#1145 so 3759aab applies the same changes