Log updates

For the most part, there is no need to include as much information in the log entries - talk to @Mullefa for more details.

Also, it'd be useful to include the test variant when logging information about ab tests e.g. [here](https://github.com/guardian/identity-processes/blob/master/payment-failure/src/main/scala/com/gu/identity/paymentfailure/BrazeEmailService.scala#L92).

Talk to @Mullefa to clarify which log entries should contain less information and how the test variant should be represented. Start with the logging around BrazeEmailService.scala line 92, then identify the affected AB-test logging and confirm that the resulting entries contain the agreed information.

Written by the indexing model from the issue text.