Replace most strpos() === 0 or !== 0 with str_starts_with()

Adding credits from the original MR. (All I did was split it up; others created it first.)

xjm → credited alexpott → .

xjm → credited longwave → .

xjm → credited catch → .

All the changes look good. Agreed with doing the easy ones here and stragglers at the end.

Automatically closed - issue fixed for 2 weeks with no activity.