Unify Seq.Syntax assembly references to the shipped version

[nblumhardt]

The 2024.1 app host is failing to load apps that depend on Seq.Syntax; we're unsure of the root cause but need to unblock the final 2024.1 release, so this PR works around the issue by resolving all Seq.Syntax assembly references to the one we ship with seqcli.

I took the opportunity to break a trivial remaining Serilog.Expressions dependency while I was here, since that's very likely the next problem we'll encounter.

Also noting that Seq.Syntax depends on the published Superpower package, while the earlier Serilog.Expressions dependency embedded this.

[KodrAus]

RUNNING SpanIngestionTestCase                             
Failed with status code BadRequest: The event on line 1 has a start timestamp `@st` but no span id.
FAIL
Xunit.Sdk.EqualException: Assert.Equal() Failure: Values differ
Expected: 0
Actual:   1

[nblumhardt]

datalust/seq-app-mail#19 is the PR tracking the upstream Seq.Syntax updates I mentioned