wrong calendar user-agent since thunderbird 78
since lightning is part of thunderbird, it sends "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0" instead of "Lightning".
not every caldav server accepts all user agents. in my case it's not possible to change it on providerside.
workaround: global.useragent.override = Lightning
new problem: every component of thunderbird sends "User-Agent: Lightning", so e.g. no Add-ons can be downloaded.
needed solution: an individual configuration to override the User-Agent of the calendar, e.g. "calendar.useragent.override"