Install flexmailer by default on new installs

It's time....

End goal is it is a required extension & the code it replaces is entirely removed

enable by default on new installs https://github.com/civicrm/civicrm-core/pull/21522
add status check and or upgrade messages when it is not installed or not being used as the delivery mechanism
consider #2898 (closed)

We should move fairly quickly from that to simply making it required IMHO since it's been around ... forever

Edited Dec 19, 2022 by totten

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