Update to Orleans 8.1.0 and .Net8

[SebastianSornsen]

Kindly consider upgrading to .Net8.0 and Orleans to version 8.1.0.
For personnal use and testing I already made quick changes in my personal SignalR.Orleans fork to allow compiling. I didn't see any problems from my preliminary, altough limited, tests.

[SebastianSornsen]

Issue #151 is still very much present and has prevented me from testing in a production environnment using the MongoDb provider in my case.

[klepersontimur-ilogos]

is there any plans to migrate on .net 8?

[EricSites]

Yea, what is the deal with this? Is SignalR for Orleans going to be support anymore?

[ranjithashetty996]

Please plan on migrating to .Net 8 soon. Also System.Text.Json vulnerability fix

[rkargMsft]

Looks like there's a PR for this waiting on review: #162