Migrate to -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 mode

[juj]

We should, with proper cadence,

1. switch to -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 mode as default build mode, and issue a warning if -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=0 mode is used,

and 2. drop support for -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=0.

Flipping the default from 0 to 1 could occur as part of 1.39.0 version bump.

[juj]

For details, see #7977.

[juj]

For more details on the mailing list, see https://groups.google.com/d/msg/emscripten-discuss/xScZ_LRIByk/_gEy67utDgAJ

[VirtualTim]

DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR was added ~10 months ago. Is it time to set the default to 1?
@kripken?

[kripken]

Good point, that does seem like a sufficient amount of time. If someone wants to drive that, it sounds ok to me.

[kripken]

Looks like the unconstructive comments have been deleted here already.

Please in the future be respectful of everyone on this issue tracker.