Remove code that provides compatibility with unsupported versions of Gradle

[wilkinsona]

We have some code in our Gradle plugin that provides compatibility with unsupported versions of Gradle. For example, in 2.5.x the minimum supported version of Gradle is 6.8 yet ApplicationPluginAction contains some logic that checks for Gradle 6.4. Similarly, JavaPluginAction contains some logic that checks for Gradle 6.7.