Replace SimpleJson usage with System.Text.Json

[danielmarbach]

Replaces the previous usage of SimpleJson in two places with System.Text.Json:

• Diagnostics file output
• Learning Persistence saga storage

We have replaced in a number of downstream the simplejson usages as well and so far, they have been frictionless. For the learning persistence I don't think we give any guarantees, so if there is a breaking change (which is quite unlikely) then that wouldn't be the end of the world since it is not a production persister.

The startup diagnostic files get overwritten on each start so that is also not a problem.

[DavidBoike]

@danielmarbach any theories on the test failures?

[danielmarbach]

One is the encoding of the diagnostic entries. The other one don't know yet

[danielmarbach]

For some reasons I couldn't figure out yet it seems we are now exposed to race problems on the saga persister.

[DavidBoike]

Saga issues were because System.Text.Json serializer does not "clip" the end of a file's output when you are writing to a read/write FileStream. You have to do fileStream.SetLength(fileStream.Position) after writing or you could have leftover characters from the previous contents of the file.

[danielmarbach]

Awesome. Nice find!