Remove legacy code and updates

[Rotzbua]

@Minishlink some task I think that should be done before a release.

• [x] There is legacy code to support early Chrome push by firebase. This is deprecated by google. This should be removed.
• [x] Nowadays every bundled openssl should support ec keys, so workaround code should be removed.
• [x] Cleanup direct use of gmp
• [x] Better check for missing extensions/functions

[Minishlink]

1. yes let's remove the legacy chrome push FCM it will become obsolete on June of this year
2. ok, is the first version of openssl that supports this known? We could add this to the readme to redirect people to older versions of web-push-php if needed
3. ok
4. ok

Thanks @Rotzbua!

[enact-on]

Hello Team ( @Minishlink @Rotzbua ),

We've received a notification from Firebase indicating that one of our legacy projects, which utilizes an older version of a particular library, is now deprecated. We need to determine if this library has been updated to align with Firebase's new API approach.

Also I could sponsor a goal if someone could help upgrade current codebase to new version.

[Minishlink]

Hello, this library never used Firebase Cloud Messaging per se. Older Chrome versions were using FCM endpoints for web push and that goes through the library transparently. More information in the README If you want to upgrade, wait for the next version that will come out later with this issue closed, as there will be some breaking changes. Note that newer versions don't support old versions of PHP (see README for details)