Replace silverstripe/react-injector with admin injector

[emteknetnz]

Follow on from silverstripe/silverstripe-mfa#584 (comment)

https://github.com/silverstripe/react-injector/ is an unmaintained repository that is something of an out of date clone of lib/Injector that is in silverstripe/admin. silverstripe/react-injector is only used by silverstripe/mfa to get the injector functionality without being logged in to admin. It does not appear to be used by silverstripe/silverstripe-login-forms.

Injector should instead be exported as a separate bundle by silverstripe/admin so that it can be consumed both inside and outside of admin.

Acceptance criteria

• Injector is exported as its own bundle in silverstripe/admin
• All existing admin functionality that relies on injector continues to work
• silverstripe/mfa is updated to use the exported bundle rather than silverstripe/react-injector
• silverstripe/react-injector is archived