Switch default for `useDefineForClassFields` in ESNext

[DanielRosenwasser]

Since this targets 3.7.1, I'm going to close this bug. When class fields reaches stage 4, I'll do the final two steps.

Originally posted by Nathan Shively-Sanders (@sandersn) in #27644 (comment)

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useDefineForClassFields should be switched to true when targeting ESNext.

[sandersn]

Class fields aren't stage 4 yet. Moving to 3.9.

Edit: Still not stage 4. I'll move to 4.4 when that milestone gets created.

[jelbourn]

This isn't mentioned in the RC notes for 3.9- has it been bumped to a later version?