Build: Explicitly test on Safari 26, not latest

BrowserStack often shares the Safari version included in the next macOS release
as soon as that macOS releases a public beta, but there's no indicator in the
API that this Safari version is a beta, making JTR pick it up via
`Safari_latest`. To avoid this issue, specify Safari versions explicitly.
Fortunately, major Safari versions are only released once a year, so the list
can usually be updated together with a related iOS yearly major update.

Closes gh-635
Ref gh-636
Ref jquery/jquery#5857
Ref jquery/jquery#5858

Author: mgol

.github/workflows/browserstack.yml

@@ -28,10 +28,10 @@ jobs:
       matrix:
         BROWSER:
           - 'IE_11'
-          - 'Safari_latest'
-            # JTR doesn't take into account the jump from Safari 18 to 26,
-            # so we need to specify versions explicitly.
-            # See https://github.com/jquery/jquery-test-runner/issues/17
+          # BrowserStack makes Safari on beta macOS versions available without marking
+          # either the OS or the browser as beta, so we have no way to resolve "latest"
+          # to the lastest stable version.
+          - 'Safari_26'
           - 'Safari_18'
           - 'Chrome_latest'
           - 'Chrome_latest-1'