upgrade gradle: v6 is at end of life

[jzacsh]

https://endoflife.date/gradle probably worth spending a bit of time to test and make sure we can upgrade. and if we're gonna do that, we might as well jump to the latest (8.x)

• our 6.x reference:

  data-transfer-project/gradle/wrapper/gradle-wrapper.properties

  Line 3 in 4a6b403

  distributionUrl=https\://services.gradle.org/distributions/gradle-6.9.2-bin.zip

• 6->7 https://docs.gradle.org/current/userguide/upgrading_version_6.html
• 7->8 https://docs.gradle.org/current/userguide/upgrading_version_7.html