[WIP] update gradle, kotlin and kotlin-dsl versions

[thatfiredev]

This PR updates some of our dependencies:

• gradle 5.6.4 --> 6.1.1
• kotlin 1.3.72 --> 1.6.20
• kotlin-dsl 1.2.9 --> 1.3.3

Main reason for this change is that it will unblock some feature requests such as #1252 while also getting the project more up to date.

[google-oss-bot]

Coverage Report ¹

Affected Products

No changes between base commit (672c6dc) and merge commit (3bd6774).

Test Logs

• Base (672c6dc): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1519376072494288899
• Merge (3bd6774): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3684/check-coverage-changed/1519448296777584640

Notes

• Commit (3bd6774) is created by Prow via merging PR base commit (672c6dc) and head commit (4614adb).
• Run gradle <product>:checkCoverage to produce HTML coverage reports locally. After gradle commands finished, report files can be found under <product-build-dir>/reports/jacoco/.

1. https://storage.googleapis.com/firebase-sdk-metric-reports/L27v6Wuu1d.html

[google-oss-bot]

@thatfiredev: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
build-plugins-check	4614adb	link	/test build-plugins-check
api-information	4614adb	link	/test api-information
check-changed	4614adb	link	/test check-changed
smoke-tests	4614adb	link	/test smoke-tests
binary-size	4614adb	link	/run binary-size
device-check-changed	4614adb	link	/test device-check-changed

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[thatfiredev]

Looks like part of this is being handled on #3744
I'll put this on hold until that PR gets merged.

[thatfiredev]

Closing in favor of #3744 and #3762