Move macOS EWS to Big Sur
rdar://87553340
https://ews-build.webkit-uat.org/#/builders/59/builds/2261 shows a successful build on Big Sur https://ews-build.webkit-uat.org/#/builders/4/builds/483 shows a successful API test run
https://ews-build.webkit-uat.org/#/builders/63/builds/377 shows successful Release WK2 test run
https://github.com/WebKit/WebKit/pull/123
Landed https://commits.webkit.org/247034@main (aaed78a2dc0472ad6ed93f59461b68fc9653ffe4)