Fixed #34832 -- Made admin's header content render in <header> tag.

[sabderemane]

Header tag was changed to <header> get the landmark banner for accessibility.

Note: I remove the overflow: hidden, here is why :

Without the overflow ⬇️

With the overflow ⬇️

[thibaudcolas]

Looking good @sabderemane! If we remove the overflow then I think we can go one step further and also remove:

django/django/contrib/admin/static/admin/css/base.css

Lines 837 to 839 in f92e68c

	#container > div {
	flex-shrink: 0;
	}

As far as I can see this only applied to the header div.

[felixxm]

@sabderemane Thanks 👍 Can you add a small test and release note?

[sabderemane]

Sure @felixxm, I'll add both :)

[felixxm]

@sabderemane Thanks 👍

I changed to a small non-selenium test.