Migrate rest.li build to use Gradle 6.9.4

[bzzzzzz]

This PR moves rest.li from using Gradle 5.2.1 to Gradle 6.9.4 to build itself. It also drops the support of all Gradle versions before 6.9.4 for the pegasus plugin. Here are the list of changes that made it possible:

• Gradle service files were automatically updated by the ./gradlew wrapper --gradle-version 6.9.4 command.
• Gradle testing framework was explicitly configured to run with integTest source set as it is required in newer Gradle version
• The list of Gradle version to use with testing was standardized in one class
• PegasusPlugin was configured to accept the PEGASUS_INTEGRATION_TESTING variable because the hack with substituting the integration testing classpath is not possible anymore in newer Gradle versions

Testing Done

• locally ran ./gradlew build and confirmed that it works