chore: Add a way to remove an invalid token if sendFCM returns 404

[Gustrb]

Proposed changes (including videos or screenshots)

The old library (node-gcm) we used to send messages via the legacy push had some utilities to help us out implementing the specification described here on how to handle error codes, and we used to functions, _removeToken and _replaceToken to minimize the number of unnecessary errors we would propagate over.
Now, we had to implement the utility to send messages manually, in #32208 now, we implemented most of the error handling utilities in the function called fetchWithRetry, but there was a big one missing, the 404 case.
From the spec:

UNREGISTERED (HTTP error code = 404) App instance was unregistered from FCM. This usually means that the token used is no longer valid and a new one must be used.

This error can be caused by missing registration tokens, or unregistered tokens.
Missing Registration: If the message's target is a token value, check that the request contains a registration token.
Not registered: An existing registration token may cease to be valid in a number of scenarios, including:

• If the client app unregisters with FCM.
• If the client app is automatically unregistered, which can happen if the user uninstalls the application. For example, on iOS, if the APNs Feedback Service reported the APNs token as invalid.
• If the registration token expires (for example, Google might decide to refresh registration tokens, or the APNs token has expired for iOS devices).
• If the client app is updated but the new version is not configured to receive messages.

For all these cases, remove this registration token from the app server and stop using it to send messages.

OBS: not adding replaceToken because the new Firebase API does not return the new device token ever, so it is better to just remove it, and let it be recreated when a new push gets sent.

Issue(s)

CONN-205

Steps to test or reproduce

You can just follow: #32208

Further comments

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.35%. Comparing base (99de6d2) to head (f940505).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #32440      +/-   ##
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- Coverage    56.42%   56.35%   -0.07%     
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  Files         2447     2450       +3     
  Lines        53977    54019      +42     
  Branches     11125    11135      +10     
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- Hits         30457    30444      -13     
- Misses       20868    20922      +54     
- Partials      2652     2653       +1     

Flag	Coverage Δ
e2e	56.16% <ø> (-0.06%)	⬇️
unit	71.90% <ø> (+0.09%)	⬆️

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