Update backtrace integration note for Swift 5.9+ in building.html

[andrehora]

Motivation:

Webpage https://www.swift.org/documentation/server/guides/building.html contains outdated information about backtracing support: "Eventually this will become a language feature and not require a discrete library.", but backtracing is built into Swift 5.9+.

See: https://github.com/swift-server/swift-backtrace

Modifications:

Updated the swift-backtrace integration note to reflect that Swift 5.9+ has built-in backtracing support.

Result:

Up-to-date information about backtracing support to users.

[heckj]

thanks @andrehora

I'm actively updating these more broadly, and setting up a PR to shift into a broader collection of Swift server guides at swiftlang/docs#31 - take a look there, I think this update is effectively represented there, but I'm always game for additional input.