[bisq-network/bisq] Bump gradle to v6.8.3, mockito to v3.9.0 (#5403)

[Maxim Belkin]

Running `./gradlew build` on macOS 10.16 BigSur produces the following error message:
```
FAILURE: Build failed with an exception.

* What went wrong:
Could not initialize class org.codehaus.groovy.runtime.InvokerHelper

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s
```

A quick Google search showed that Gradle 6.3 or above it required. This new Gradle, in turn, requires new(er) Mockito. In this PR I bump both to the latest stable versions. With these fixes, I can successfully execute `./gradlew build` on macOS BigSur.
You can view, comment on, or merge this pull request online at:

  https://github.com/bisq-network/bisq/pull/5403

-- Commit Summary --

  * Bump gradle to v6.8.3, mockito to v3.9.0

-- File Changes --

    M build.gradle (2)
    M gradle/wrapper/gradle-wrapper.properties (4)

-- Patch Links --

https://github.com/bisq-network/bisq/pull/5403.patch
https://github.com/bisq-network/bisq/pull/5403.diff

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