[Mastodon] Jetpack Social Connection fails to integrate with Mozilla.social

### Impacted plugin

Social

### Quick summary

The Mastodon integration for Jetpack is super, super useful, and helps us auto-post our articles out to our Mastodon followers. We moved from Mastodon.social to Mozilla.social, as we believed this to be a positive thing for our brand. However, Mozilla.social never allows for the connection to be completed. It appears that one complication may come from how Mozilla is doing account management, as users must sign in to the instance using Mozilla Accounts.

### Steps to reproduce

1. Start at https://wordpress.com/marketing/connections/youdomain.tld (alternatively, use the Jetpack for Mobile app and find the equivalent page)
2. Click "Connect" next to the Mastodon option.
3. Enter any Mastodon username in the box at the mozilla.social domain. In our case, @wedistribute@mozilla.social
4. The OAuth page for Mozilla.Social will appear with error details.

### A clear and concise description of what you expected to happen.

When connecting to any Mastodon instance, it is expected that the pop-up window presents the Mastodon OAuth page for the app. Upon accepting, the window is supposed to close, and the integration connection is completed.

### What actually happened

The Mozilla.Social OAuth page, normally used for approving the app permissions, is displayed with the following error message:
> Client authentication failed due to unknown client, no client authentication included, or unsupported authentication method.


### Impact

One

### Available workarounds?

No and the platform is unusable

### Platform (Simple and/or Atomic)

_No response_

### Logs or notes

This happens both on web and Android app versions of Jetpack, when attempting to connect to Mozilla.social. Other servers work fine, so I believe the problem may boil down to either Mozilla's own account management system, or a lack of whitelisting Jetpack as a viable client. Still, it's worth testing and verifying a fix.