Excavator: Upgrade to Gradle 8

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[schlosna]

@CRogers flagging this for SA

The build is failing on metric-schema's generate-metrics task not being compatible with gradle 8, so we probably need to address that & palantir/metric-schema#1089 first.

> Task :tritium-lib:sourcesJar FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Some problems were found with the configuration of task ':tritium-lib:sourcesJar' (type 'Jar').
  - Gradle detected a problem with the following location: '/home/circleci/project/tritium-lib/build/metricSchema/generated_src'.
    
    Reason: Task ':tritium-lib:sourcesJar' uses this output of task ':tritium-lib:generateMetrics' without declaring an explicit or implicit dependency. This can lead to incorrect results being produced, depending on what order the tasks are executed.
    
    Possible solutions:
      1. Declare task ':tritium-lib:generateMetrics' as an input of ':tritium-lib:sourcesJar'.
      2. Declare an explicit dependency on ':tritium-lib:generateMetrics' from ':tritium-lib:sourcesJar' using Task#dependsOn.
      3. Declare an explicit dependency on ':tritium-lib:generateMetrics' from ':tritium-lib:sourcesJar' using Task#mustRunAfter.
    
    For more information, please refer to https://docs.gradle.org/8.5/userguide/validation_problems.html#implicit_dependency in the Gradle documentation.
  - Gradle detected a problem with the following location: '/home/circleci/project/tritium-lib/build/metricSchema'.
    
    Reason: Task ':tritium-lib:sourcesJar' uses this output of task ':tritium-lib:compileMetricSchema' without declaring an explicit or implicit dependency. This can lead to incorrect results being produced, depending on what order the tasks are executed.
    
    Possible solutions:
      1. Declare task ':tritium-lib:compileMetricSchema' as an input of ':tritium-lib:sourcesJar'.
      2. Declare an explicit dependency on ':tritium-lib:compileMetricSchema' from ':tritium-lib:sourcesJar' using Task#dependsOn.
      3. Declare an explicit dependency on ':tritium-lib:compileMetricSchema' from ':tritium-lib:sourcesJar' using Task#mustRunAfter.
    
    For more information, please refer to https://docs.gradle.org/8.5/userguide/validation_problems.html#implicit_dependency in the Gradle documentation.

[schlosna]

Created palantir/metric-schema#1092 to remove need for this