Optimization: used StartsWith(char) instead of StartsWith(string) where supported

[epeshk]

This PR optimizes the SerilogLogger.PrepareWrite method by using a StartsWith(char) overload instead of StartsWith(string) on >= net6.0, >= netstandard2.0 platforms.

Big win in the benchmark from NLog project (https://github.com/NLog/NLog-Performance-tests/blob/master/MicrosofLoggingPerformance/Program.cs)

Before

Test Name	Time (ms)	Msgs/sec	GC2	GC1	GC0	CPU (ms)	Alloc (MB)
NLog	4 895	1 021 447	0	0	13	4 109	2 021,8
Serilog	17 058	293 116	0	0	22	12 468	3 474,5

After

Test Name	Time (ms)	Msgs/sec	GC2	GC1	GC0	CPU (ms)	Alloc (MB)
NLog	4 753	1 051 935	0	0	13	4 109	2 021,8
Serilog	3 443	1 452 118	0	0	22	3 109	3 514,8

[shvez]

@snakefoot fyi

[Numpsy]

That looks like a pretty large gain for a simple change :-)

Does it have any effect on the local benchmarkdotnet project? (there were some other perf changes going on for that in #247 )

[epeshk]

In #247 this issue is addressed with StringComparison.Ordinal, which is optimized on modern runtimes

[nblumhardt]

I like the StringExtensions polyfill; nicer to use the more modern APIs 👍

I'll merge and resolve conflicts with the other PR. We'll need to release a 9.x of this library fairly soon, but an 8.0.1 with this could also be worthwhile, given the throughput boost :-)