Boost 1.87 example update

[anarthal]

In 1.87, any_connection and SQL formatting are stable, so examples should be updated to reflect these new best practices. Now that C++20 coroutines are more widespread and that we support them cleanly thanks to with_diagnostics, most examples should be updated to use these. Some need to be reviewed (as they were written long ago).

I propose having simple-but-complete examples demonstrating a single feature (e.g. suppressing exceptions, using pipelines, avoiding copies in SQL formatting, metadata...) plus a couple of fully-fledged web servers. In particular, I dislike the current examples about orders, because they don't fit any of these. Some ETL-like examples would be nice, too.