Add wgReadingListsBetaDefaultForNewAccountsAfter to opt new accounts into beta feature

We will be opting new accounts into the ReadingLists beta feature on web, as part of phase 2 of WE3.3.4/3.8.2 Reading List Experiment & Rollout.

The beta cluster already has this setting and would help if patchdemo matches.

Bug: T419163

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