use same nuget key as main serilog repo

[SimonCropp]

@nblumhardt is there a reason why the nuget keys are different on the diff repos?

[nblumhardt]

I think the core Serilog package needs/uses a different key from the sinks/extensions because (historically) the contributor/maintainer group for the core was much more limited than for the additions and this helped firewall the supply chain just a little bit more :-)

Right now, this will fail if merged, but given the much smaller Serilog org we now have, it would make sense to either use a single key across them, or even keep a key per repository 🤔

[nblumhardt]

(Marking as draft helps avoid an accidental later merge :-))

[SimonCropp]

@nblumhardt i only did this since current key has expired https://ci.appveyor.com/project/serilog/serilog-framework-logging/builds/46479990

can u gen a new key and update the one stored in appveyor

[nblumhardt]

@SimonCropp thanks 👍 - EN9f+XXE3fW+ebL4wxrIbafdtbNvRfddBN8UUixvctYh4qMBHzr1JdnM83QsM1zo should do the Extensions.* projects for another year.

It's a bit of an annoying system to maintain; we're moving to another approach for work projects (use an agent to harvest GitHub releases and centrally update NuGet etc.), it might be the way to go for Serilog eventually, too.