Now that master is 8.0, we can remove uses of these constants and the backcompat code that uses them, since 8.0 will always walk to 7.0+ nodes.

This issue is for tracking removing uses of the constants.

• v6.0.0 prerelease versions (Remove Version 6 pre-release constants #41517)
• v6.0.x (Remove 6.0 version constant uses #41965)
• v6.1.x (remove backcompat handling of 6.1.x versions #42032)
• v6.2.x (remove backcompat handling of 6.2.x versions #42044)
• v6.3.x (remove 6.3.x constants #42087)
• v6.4.x (remove 6.4.x version constants #42127)
• v6.5.x (remove v6.5.x and v6.6.x version constants #42130)
• v6.6.x (remove v6.5.x and v6.6.x version constants #42130)
• v6.7.x (Remove the 6.7 version constants. #42039)
• v6.8.x (remove v6.8.x version constant and the backcompat code that uses it #42146)