Misc: Drop support for older browsers; update support comments

That includes IE<8, Opera 12.x, Firefox<29, Safari<6.0 and some hacks
for old Blackberry.

Fixes gh-1836
Fixes gh-1701
Refs gh-1815
Refs gh-1820
Noticed in passing, but can we get better bounds on this support range and transfer its documentation to [attributes/support.js](https://github.com/mzgol/jquery/blob/drop-old/src/attributes/support.js#L28-L30)?