fix(dav): Try basic auth for ajax WebDAV requests

[susnux]

Summary

Allow using basic auth with ajax requests on WebDAV.
The idea why the code was added in the first place was to ensure that if basic auth fails the user gets not asked by the browser for the correct credentials.
We still keep this behavior by throwing a 401 error in case of failed basic auth.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

The problem gets visible because previous versions of @nextcloud/axios were not setting the header, but the latest version will set the X-Requested-With header by default.

Meaning the alternative to this is to drop the X-Requested-With: XMLHttpRequest from axios requests to DAV like we were doing with current versions (currently we do not set X-Requested-With for ajax requests to DAV to prevent this.
So I think this solution here is cleaner than faking the kind of request 😅

[nickvergessen]

Blocking temporarily

[github-actions]

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Tested and concerned issue is not happening

[sorbaugh]

/backport to stable29