Mark sandbox="allow-downloads" support in Safari (#23951)

It landed in the WebKit main branch here:
WebKit/WebKit@f8f6da2

However, Originally-landed-as point to this commit:
WebKit/WebKit@9d46b77
https://github.com/WebKit/WebKit/blob/9d46b7777bca2b36c82f040065e3d7953f8f782d/Configurations/Version.xcconfig

Going by the date (2023-07-13) and WebKit version (616.1.24) Safari 17 is the most likely correct version.

It's not mentioned in the release notes, and no testing was done to confirm support.

html/elements/iframe.json

@@ -753,7 +753,7 @@
                 "opera": "mirror",
                 "opera_android": "mirror",
                 "safari": {
-                  "version_added": false
+                  "version_added": "17"
                 },
                 "safari_ios": "mirror",
                 "samsunginternet_android": "mirror",