fix(popover): default alignment to 'center' for ios mode

[averyjohnston]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Popovers always default to alignment="start". This causes problems on ios mode when the trigger element is much smaller than the popover; the arrow falls outside the trigger element.

Issue URL: Resolves #24389

What is the new behavior?

Popovers' alignment now defaults to "center" on ios mode only. This matches the Ionic v5 behavior.

Does this introduce a breaking change?

• Yes
• No

While changing the default value of alignment is technically a breaking change, this also technically fixes a regression from v5 where the defaults were as this PR makes them.

Other information

There is another PR at #24813 that aimed to have the arrow always point at the center of the trigger element regardless of alignment, but the team decided it added too much complexity for what is ultimately a narrow use case.