[gnrt] `guppy` prep: Explicitly add `icu_capi`'s `experimental` feature.

The `icu_experimental` crate is used in some local experiments.  Before
this CL, the `experimental` feature of `icu_capi` crate is not enabled.
So before this CL there was a risk of removing/unvendoring the
`icu_experimental` crate (when `gnrt` gains better understanding of
`cargo` dependency graphs after the `guppy` migration).

To avoid the risk above, this CL explicitly adds/enables `icu_capi`'s
`experimental` feature in `chromium_crates_io/Cargo.toml` and then
reruns `tools/crates/run_gnrt.py vendor` (no-op) and `.../run_gnrt.py
gen` (reflecting the change in `icu_capi/v2/BUILD.gn`).

Bug: 395924069
Change-Id: I1d6d434c9ae6b81db155a89a121a156706a81a54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6333243
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431033}