Prepare switch to mozilla87

• #40361 (closed) (Rebase tor-browser patches to 87.0b4)
• tor-browser-build#40241 (closed) (Update components for switch to mozilla87-based Fenix)
• #40362 (closed) (Review developer notes for Firefox 87)
• #40363 (closed) (Review GeckoView Changelog for gv87)
• #40364 (closed) (Review closed mozilla87 bug)
• #40365 (closed) (Rebase 10.5 patches on 87.0)
• android-components#40043 (closed) (Rebase android-components patches for Fenix 87 beta X builds)
• android-components#40044 (closed) (Review MozAC Changelog for Fenix87)
• fenix#40150 (closed) (Rebase Fenix patches to Fenix 87 beta 2)
• tor-browser-spec#40016 (closed) (FF87 network audit)
• tor-browser-bundle-testsuite#40019 (closed) (Update tests when switching to Fenix 87)
• fenix#40152 (closed) (Review changes for Fenix87)