Add setRequestMetadata, update iOS deployment target and remove web implementation

[DanProudfoot]

Hi - shout in advance if all three of these at once is a bit too much, it's just an offshoot of work I've done personally to get this plugin to where I need it. I've changed:

• Added setRequestMetadata(key, value) to allow integrations like Mixpanel to work correctly.
• Updated iOS deployment target to 12.0, as Capacitor 3 doesn't run if a Pod is targeting a lower version
• Completely remove the web implementation as it was causing false positives when running Capacitor.isPluginAvailable("BranchDeepLinks") in the browser environment. As all that the web implementation does is reject on every function call, I feel this is a more elegant solution. I took inspiration from @capacitor/keyboard which works in the same way.

This is my first pull request on anything open source...ever, so please let me know if I need to make tweaks or anything of the sort

[selected-pixel-jameson]

Really need to get this released.

[skout90]

@DanProudfoot your code helped me, Thanks 🙏