Drop support for IE11 and multi-target builds

[scinos]

Task Details
We want to drop support for IE11. It will allow us to drop support for multi-target builds, simplifying our code and (in theory) making the build process faster.

Drop support for IE 11:

• Implement redirect to /browsehappy for fallback browsers (server: Redirect fallback browsers to /browsehappy #53221)
• Ensure that the production browsehappy page routes to Calypso instead of Atlas (done in pMz3w-dyS-p2)
• Enable fallback redirect in production (Goodbye, IE 11 #55142)
• Revert fallback redirect (Revert "Goodbye, IE 11 (#55142)" #55317)
• Support calypso.live in desktop app CI (important for ensuring it doesn't break again) (desktop: use calypso.live for desktop e2e #55323)
• Implement a more secure redirect mechanism (Browsehappy: More secure redirect #55402)
• Make sure the desktop app is a supported browser (Add electron to browserlist #55404)
• Enable logging for the fallback page so that we know what % of requests we consider unsupported (Browsehappy: Record tracks event when redirecting #55732)
• re-enable fallback redirect in production. (Redirect unsupported browsers #55775)

Drop support for multi-target builds:

• Do not serve fallback build to redirected (unsupported) agents (server: Serve all requests using evergreen #56029)
• Remove nomodule redirect backup (done in server: Serve all requests using evergreen #56029)
• Do not build target fallback anymore (ci(build): Drop fallback build #53366)
• build: Drop fallback support in calypso-polyfill #56198
• build: Simplify generation of assets list #56195
• Run build-client instead of build-client-both #56127
• Rename target evergreen to something more sensible (eg: app)
• Drop references to "target build" or "fallback build"
• Remove fallback redirect environment feature flag

Other Tasks

• Delete the legacy browsehappy page from atlas (D65139-code)
• Make sure that when we redirect the desktop app to "unsupported browser" page, we show a more helpful message (like to update the app).