Fix login with Gitlab (change API `Authorization` header "token" → "Bearer")

[floscher]

The authorization type "Bearer" is more widely recognized than "token". E.g. Gitlab requires that you use "Bearer".

Github mentions that it accepts both "token" and "Bearer": https://docs.github.com/en/rest/authentication/authenticating-to-the-rest-api?apiVersion=2022-11-28#about-authentication

Gitlab seems to require the use of "Bearer", which was apparently the cause of #7172: https://github.com/decaporg/decap-cms/issues/7172#issuecomment-2184138768 (thanks to @b-xb for digging into this)

See also: https://github.com/decaporg/decap-cms/pull/5844#discussion_r1658974332

Fixes #7172.

Summary

Gitlab login is broken in current versions: #7172

Test plan

Checklist

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