Update Gradle wrapper to 8.6

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Description

Update Gradle distribution version & regenerate wrapper to pick up recent changes per https://docs.gradle.org/current/userguide/gradle_wrapper.html#sec:upgrading_wrapper

./gradlew wrapper --gradle-version 8.6
./gradlew wrapper --gradle-version 8.6

https://docs.gradle.org/8.6/release-notes.html

Motivation and Context

Potential fix for https://github.com/dita-ot/dita-ot/issues/4426, which reports errors when running the currently included Gradle wrapper version 7.6.1 with Java 21.

How Has This Been Tested?

• Created fresh worktree in core repo
• Updated Gradle wrappers in both core and docs repos to version 8.6
• Tested with Java 17 and Java 21

Results

• ✅ Java 17 builds completed successfully
• ❌ Java 21 builds throw implicit dependency errors (as if html and pdf tasks each depend on one another — even though they don't).

Show Gradle error details

> Task :pdf FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Some problems were found with the configuration of task ':pdf' (type 'DitaOtTask').

  - Gradle detected a problem with the following location: 
    '/…/dita-ot.dist.worktree.g86.j21/build/tmp/dist/doc'.
    
    Reason: Task ':html' uses this output of task ':pdf' 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 ':pdf' as an input of ':html'.
      2. Declare an explicit dependency on ':pdf' from ':html' using Task#dependsOn.
      3. Declare an explicit dependency on ':pdf' from ':html' using Task#mustRunAfter.
    
    For more information, please refer to 
    https://docs.gradle.org/8.6/userguide/validation_problems.html#implicit_dependency 
    in the Gradle documentation.

  - Gradle detected a problem with the following location: 
    '/…/dita-ot.dist.worktree.g86.j21/build/tmp/dist'.
    
    Reason: Task ':pdf' uses this output of task ':html' 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 ':html' as an input of ':pdf'.
      2. Declare an explicit dependency on ':html' from ':pdf' using Task#dependsOn.
      3. Declare an explicit dependency on ':html' from ':pdf' using Task#mustRunAfter.

Note To test, the wrappers must be updated in both core & docs repos. See corresponding PR in core repo: https://github.com/dita-ot/dita-ot/pull/4446.

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Gradle 8.7 was released the same day I started testing this.

Will repeat tests with the latest version.