Flexmailer - Enable traditional mailing support by default

[eileenmcnaughton]

Overview

Switches to defaulting to flexmailer, if enabled.

This should be merged in conjunction with #19806
in order to keep those on the default in consistent state
(ie they will already be getting display-rendering for custom fields
and that will still be the case if 19806 is merged)

Before

If not otherwise set flexmailer defaults to using Mailing_BAO classes

After

If not otherwise set flexmailer defaults to flexmailer class

Technical Details

Step towards only having one code path to maintain

Comments

[civibot]

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[totten]

This is a good idea and agree with linking it to #19806.

One small observation - the idea behind default=>auto is that the sysadmin specifically defers to upstream. I used this pattern because I expected we would want to flip the default at some point. Right now, the effective interpretation of auto is "Use bao". But we could change it to mean "use flexmailer".

If the only concern is new installations, then default=>flexmailer and default=>auto[aka flexmailer] are the same. The difference is for existing sites that are aligned with the default. (You might call these agnostic configurations.) Depending on utter happenstance, some agnostics have blank/omitted civicrm_setting (ie they track the metadata-default) while others have civicrm_setting persisted with the old default (so they don't track metadata-default).

IMHO, default=>flexmailer maybe makes sense if the intent is to leave some agnostics on bao. But if the idea is to flip all agnostics to flexmailer, then default=>auto[aka flexmailer] would be more effective.

[eileenmcnaughton]

@totten OK - I think switching auto over makes more sense - where would we make that change

[totten]

@eileenmcnaughton Since we've merged the token fix, I've pushed up the revision for how to flip the interpretation of auto.

[eileenmcnaughton]

Looks good - this is now Tim's change & I am the review so MOP