Simplify new nav for old IE basic styling

[janbrasna]

If this changeset needs to go into the FXC codebase, please add the WMO and FXC label. 🤷 (yes, this will need re-applying to fxc 🔥🦊 with different paths)

One-line summary

Streamlines the experience for IE9- and polishes a bit more in IE8- to make the site usable with recent (mostly m24) styling additions.

Significant changes and points to review

Constrains refreshed header & footer to just top level nav items without any interaction, for better usability and scanability of the rest of pages.

Improves some basic alignment and adds alternatives for key SVG affordances (to not look broken in places where it is used for navigation/interaction, removing large objects with rendering issues like header flag or footer wordmark, augmenting download steps with plaintext pseudo content to allow navigation etc., while keeping product logos in menus for IE versions that can support that).

Removes fallback styling overrides ("enhancements") that try to inject full modern bundles (incl. current base m24 styles) for otherwise base-less legacy-only pages like fx/desktop, as it was only making the experience worse, not better. Being covered by the common-IE bundle like everywhere else, with the added extra desktop/ie* still for enhanced styling of the components there, seems to serve such clients better, also being more stable and predictable (=frozen in time, instead of blindly serving full modern bundles' changes over time…)

Also cleans up some superfluous meta and duplicate hyphenation for ?xv=basic e.g. fx/mac or fx/linux for everyone while at it… SERPs strip what's after the hyphen anyway, or use the shorter opengraph content…

Tweaks the ESR/unsupported CTAs as these are now the default for such platforms+UAs.
(+tiny RTL fixes.)

(Also, with the switch to ESR and ESR download button macro magic, these UAs skip the download/thanks flow anyways and request direct bouncer links, so no need to keep enhancing paths not being taken any longer.)

Removes custom IE8-9 styles that no longer need to be styled, being covered by the common-ie base protocol tweaks sufficiently.

NB: Updates browser compatibility info in docs with some more precise datapoints.

Issue / Bugzilla link

Fixes #15867

Testing

http://localhost:8000/en/
http://localhost:8000/fy/firefox/new/
http://localhost:8000/en/firefox/new/?xv=basic
http://localhost:8000/cy/firefox/windows/
http://localhost:8000/en/firefox/download/thanks/?xv=basic
http://localhost:8000/sco/firefox/download/thanks/
http://localhost:8000/he/firefox/all/desktop-esr/win/
http://localhost:8000/ar/about/
http://localhost:8000/el/ (og home)
http://localhost:8000/pl/ (og nav)

🚧 TKTK fresh screen captures

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.57%. Comparing base (cef4b3d) to head (d5c86eb).
Report is 63 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15874      +/-   ##
==========================================
+ Coverage   79.28%   79.57%   +0.28%     
==========================================
  Files         159      160       +1     
  Lines        8343     8396      +53     
==========================================
+ Hits         6615     6681      +66     
+ Misses       1728     1715      -13     

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