[roll third_party/pigweed] bazel: Remove shallow_since attributes

Using shallow_since is not recommended, and the attribute is expected
to be removed in a future Bazel version:
bazelbuild/bazel#17356,
bazelbuild/bazel#12857

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/193473

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 371015e36501c36..9a3bb012f2c6dc4
Roller-URL: https://ci.chromium.org/b/8755460054149076049
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8952a6aa062170a57fca6a2f0420e40d7242d403
Reviewed-on: https://pigweed-review.googlesource.com/c/example/echo/+/193492
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>