Bump minimum Safari version in Grade C to Safari 11

The summary of browser grades include Safari 10.1 (macOS) and 10.3 (iOS) as examples of Grade C browsers.

However in the Grade C section of the document it's mentioned that Safari 10.1 will not run our JavaScript as (although it technically supports `type="module"`) it fails the prerequisite support tests present in Frontend. That section lists Safari 11 (macOS) as being the minimum for Grade C instead.

This is confusing, as different parts of the document use different Safari versions as the minimum supported version for Grade C. It also does not mention that Safari 10.3 (iOS) is subject to the same issues as Safari 10.1 (macOS).

This commit updates the summary at the top of the document and in the Grade C section to use Safari 11 as the minimum supported version, as this is what appears to be the case to a person quickly glancing at the document in reference. The Grade C section has been updated to make it clear that, while Safari 10.1 (macOS) and Safari 10.3 (iOS) are technically included, they do not actually initialise any of Frontend's components due to failing other prerequisites outside of having support for modules.

docs/contributing/browser-support.md

@@ -15,7 +15,7 @@ You can view a summary of [the grades and the support we provide](https://fronte
 
 - **grade A** - Most recent stable versions of Chrome, Firefox, Edge, Samsung Internet and Safari
 - **grade B** - All stable versions of Chrome, Firefox and Edge released in the last 6 months and all the major stable releases of Safari which are not supported in Grade A
-- **grade C** - [All browsers that support `<script type="module">`](https://caniuse.com/es6-module) (Chrome 61+, Edge 16-18, Edge 79+, Safari 10.1+ (mac), Firefox 60+, Opera 48+, Safari 10.3+ (iOS), Samsung Internet 8.2+)
+- **grade C** - [All browsers that support `<script type="module">`](https://caniuse.com/es6-module) (Chrome 61+, Edge 16-18, Edge 79+, Safari 11+, Firefox 60+, Opera 48+, Samsung Internet 8.2+)
 - **grade X** - All other browsers (including IE11 and older)
 
 > **Note: Only browsers in grades A, B and C will run our JavaScript enhancements. We will not support our JavaScript enhancements for older browsers in grade X.**
@@ -102,13 +102,12 @@ This grade covers browsers not in Grade A or B which support <script type="modul
 - Chrome 61+
 - Edge 16-18
 - Edge 79+
-- Safari 11 (mac)
+- Safari 11+
 - Firefox 60+
 - Opera 48+
-- Safari 10.3+ (iOS)
 - Samsung Internet 8.2+
 
-Safari 10.1 also supports `<script type="module">` but will 'exit early' as it does not support `HTMLScriptElement.prototype.noModule` which is how we test support for `<script type="module">` from within our JavaScript.
+Safari 10.1 (macOS) and Safari 10.3 (iOS) support `<script type="module">` but will 'exit early' as they does not support `HTMLScriptElement.prototype.noModule`, which is how we test support for `<script type="module">` from within our JavaScript. GOV.UK Frontend component JavaScript will not run in these versions.
 
 For support we will: