Fix 562, Rust example 07: vendoring

[marvin-hansen]

This PR resolves issue #562

For once, the affected Build file has been updated to use the vendored Tokio crate alias without a version number
so in case the vendoring script ever downloads a different version the build should still succeed as long as the newer
crate version remains API compatible.

Then, the root cause has been eliminated by pinning the crate dependency to an exact version so that
Bazel should download exactly the pinned version.

The issue was caused by a very unlucky version convention in crate.spec in which

'version = "1.43.0",'

does not actually pin the version. Instead, it sets a lower version bound akin to "no lower than this version",
which causes all sorts of problems. Instead, the correct form to pin a crate version is to use an equal sign prefix:

'version = "=1.43.0",

Complicating things further, the vendor script generates two aliases, one with a version and one without a version for each vendored crate.

As none of this is obvious let alone intuitive, I have updated the Readme to explain the intricate details
of the rules_rust vendoring behavior so that the reader can make an informed decision about vendoring, version pining,
and aliasing.

Housekeeping:
Bumped up Bazel version to latest 8.1.1 for all Rust examples via the .bazelversion file.

[meteorcloudy]

Thanks!

[meteorcloudy]

I guess we could revert #559

@sgowroji