Clean up incomplete migration guides tracking issue

[ElliottjPierce]

At a glance, the following migration guides need work:

• 18319 - needs consolidation and more detail.
• 17687 - blank
• 17645 - remove template
• 17581 - potentially remove
• 17567 - suspected incomplete
• 17215 - blank
• 16942 - blank
• 16911 - needs detail
• 16401 - blank
• 16279 - blank
• 16088 - blank
• 16082 - needs cleanup
• 16069 - blank
• 16050 - blank
• 15844 - blank
• 15482 - potentially redundant

I'll start working on these, but I wanted to make a record of it. Other migration guides look ok at a glance, but some may need to be revisited or consolidated. Feel free to add to the list as you find ones that need work.

[ElliottjPierce]

The rest of these need background knowledge that I don't have, but I'm glad to have made a dent. The rest of the guides look good at a glance. Once we get the migration guide page generated, I'll read over it in context and make edits.

[BD103]

bevyengine/bevy#16279 can be deleted, as it was cherry-picked into 0.15! I'll make a PR for that :)

[BD103]

Note that bevyengine/bevy#17215 has two migration guides currently. One is named "Improved Command Errors", which is blank, and the other is named "EntityCommands::apply() now takes an EntityWorldMut argument", which is complete. Does the PR have any more breaking changes beyond the EntityWorldMut argument?

Edit: I ran cargo-semver-checks to find that there was only 1 additional breaking change, which I've documented in #2079. I've merged the two migration guides together!