page-break-{before,after}: avoid not supported in Safari

This came up in the course of defining page break features in
web-features. caniuse reports partial support due to lack of support in
Safari and Firefox.

See also:

- web-platform-dx/web-features#331
- Fyrd/caniuse#7045 (comment)
- https://caniuse.com/css-page-break

css/properties/page-break-after.json

@@ -123,7 +123,8 @@
                 "version_added": "12"
               },
               "firefox": {
-                "version_added": "1"
+                "version_added": false,
+                "impl_url": "https://bugzil.la/775617"
               },
               "firefox_android": "mirror",
               "ie": {
@@ -133,7 +134,8 @@
               "opera": "mirror",
               "opera_android": "mirror",
               "safari": {
-                "version_added": "1.2"
+                "version_added": false,
+                "impl_url": "https://webkit.org/b/34155"
               },
               "safari_ios": "mirror",
               "samsunginternet_android": "mirror",

css/properties/page-break-before.json

@@ -123,7 +123,8 @@
                 "version_added": "12"
               },
               "firefox": {
-                "version_added": "1"
+                "version_added": false,
+                "impl_url": "https://bugzil.la/775617"
               },
               "firefox_android": "mirror",
               "ie": {
@@ -133,7 +134,8 @@
               "opera": "mirror",
               "opera_android": "mirror",
               "safari": {
-                "version_added": "1.2"
+                "version_added": false,
+                "impl_url": "https://webkit.org/b/34155"
               },
               "safari_ios": "mirror",
               "samsunginternet_android": "mirror",