Remake PRs targeting 0.14 branch

[alice-i-cecile]

Some PRs can't be trivially cherrypicked by @mockersf to the 0.14 release due to merge conflicts. These are:

• Ensure that events are updated even when using a bare-bones Bevy App #13808
• Generalised ECS reactivity with Observers #10839 (not yet merged)

To fix each of these, redo their changes, branched off of https://github.com/bevyengine/bevy/tree/release-0.14.0 and then make a PR on the bevyengine/bevy repo targeting the release branch.

[alice-i-cecile]

This issue mostly exists to make sure we don't forget to do this, but help here would be lovely.

[mnmaita]

I suppose this can be closed now.

[alice-i-cecile]

Yep! Thanks again for tackling this: it really helped Francois and I move the release forward.