Update toolchain setup instructions

[keith]

The big change here is that bazel 7.x shipped with toolchain resolution
flipped, so that no longer requires any configuration.

[brentleyjones]

So we don't need this anymore for Bazel 7? What changed there?

build--apple_crosstool_top=@local_config_apple_cc//:toolchain
build--crosstool_top=@local_config_apple_cc//:toolchain
build--host_crosstool_top=@local_config_apple_cc//:toolchain

[keith]

default flag flip for toolchain resolution

[brentleyjones]

Should we mention that people need to set those still if they flip the flag off?

[keith]

i left it out to try and reduce the instruction count making the assumption that those people are "on their own", especially since on rolling that flag is already a no-op, but I can add it if you think we should