Update content.json- Safari alt_text to "preview" (#21993)

* Update content.json I am updating safari `alt-text` to "preview"

I am updating `alt-text` to preview because STP 187 adds support for `alt_text`.

[STP 187 release notes](https://webkit.org/blog/14931/release-notes-for-safari-technology-preview-187/)

[Commit](WebKit/WebKit@1b3e497]

* Update content.json

css/properties/content.json

@@ -60,7 +60,7 @@
               "opera": "mirror",
               "opera_android": "mirror",
               "safari": {
-                "version_added": false
+                "version_added": "preview"
               },
               "safari_ios": "mirror",
               "samsunginternet_android": "mirror",