Migrate from Serilog.Filters.Expressions to Serilog.Expressions

[nblumhardt]

Local log filtering features are currently driven by https://github.com/serilog/serilog-filters-expressions, which is deprecated. The replacement is https://github.com/serilog/serilog-expressions, with a mostly-similar syntax, but we'd need to do some work to get property names like @Level to line up.