Update developer Slack invitation link to the one we control

[arunsathiya]

Overview

We now have a Slack invitation link for the developer Slack on a domain we control. This PR updates the links:

• Old link: https://join.slack.com/t/1password-devs/shared_invite/zt-1z0rd7lro-fx4LPkQwUF89~PzpDGPe1A
• New link: https://developer.1password.com/joinslack

Type of change

- [ ] Created a new plugin - [ ] Improved an existing plugin - [ ] Fixed a bug in an existing plugin - [x] Improved contributor utilities or experience ## How To Test

• Visit the new link
• See that it offers a sign up flow and verify that any domain can be used for the sign up.

Changelog

Update developer Slack invitation link to the one we control