Administration: Fix alignment in upload plugin/theme buttons.

Following [61598], the upload button for plugins and themes could cover the file information in the file input for longer internationalized strings due to absolute positioning.

Update layout to use remove absolute positioning while retaining the larger drop area.

Props audrasjb, pratiknawkar94, huzaifaalmesbah, noruzzaman, presskopp, joedolson.
Fixes #64832.

git-svn-id: https://develop.svn.wordpress.org/trunk@62090 602fd350-edb4-49c9-b593-d223f7449a82