Set default layout.axis.tickmode to 'sync' when axis is overlaying

[emilykl]

Closes #7683

This PR slightly modifies the default tickmode logic, such that tickmode defaults to 'sync' rather than 'auto' for overlaying axes. Motivation is explained in the linked issue.

Note: One case is excluded: When the overlaying axis is categorical (or multicategory), tickmode continues to default to 'auto' rather than 'sync', as 'sync' causes unexpected behavior with categorical axes and is probably never intended by the user.

[camdecoster]

Looks good! I left a few small syntax suggestions, but it's good to go as is.

[archmoj]

Hi @emilykl,
Thank you for the PR. Please note that the draftlog for this change is missing.
I'm wondering this PR could/should be considered a breaking change for v3.
If you are planning to release a major version, then this PR may belong to v4.
cc: @alexcjohnson

[emilykl]

@archmoj Thanks for the catch -- I've added a draftlog!

It's a good point that this is a backward-incompatible change and so strictly speaking should only be introduced in a major version. I'm not psyched about bumping the version number just for this change, but perhaps there are a few other changes we could introduce at the same time. On the other hand, do you know if there are instances in the past where we've made an exception and introduced a change in a minor version?