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

[jzacsh]

edit: done in #1400:

(overloading this issue) should probably also upgrade codeql (also past EOL) per steps in https://github.blog/changelog/2023-01-18-code-scanning-codeql-action-v1-is-now-deprecated/#what-do-i-need-to-change-in-my-workflow per this output in our ci/cd output:

Error: This version of the CodeQL Action was deprecated on January 18th, 2023, and is no longer updated or supported. For better performance, improved security, and new features, upgrade to v2. For more information, see https://github.blog/changelog/2023-01-18-code-scanning-codeql-action-v1-is-now-deprecated/