Media: make the inline uploader button a real button

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In [32467] the inline uploader button in the media template was changed in a real <button> element for better accessibility and semantics. By the way, wp.media.view.UploaderWindow still creates and appends to the body a link that, after some replacements that happen in wp.media.view.UploaderInline, is then used in the uploader.

Basically, the "Select Files" control is still a link, thus defeating the purpose of [32467]:

Since non-link links should be buttons (see #26504 and the related tickets), this should be changed in a <button> element.

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39305.diff​ makes the button a real button, no other changes.

[afercia]

In 40021:

Accessibility: Make the inline uploader button a real button.

See wp.media.view.UploaderWindow.

Fixes #39305.