[roll third_party/pigweed] pw_software_update: Update TUF bundle proto

Remove the snapshot from the Manifest and add additional comments
on proper metadata use for project-specific information.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60321

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: f6c3a78788e7cb8..7b357427a7f6578
Roller-URL: https://ci.chromium.org/b/8836530642720098177
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8836530642720098177
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9377d0a8c737be3cb104cc664c11f128b9396ef4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/60444
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index f6c3a78..7b35742 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f6c3a78788e7cb8fe094f4042e9e26a7559610bc
+Subproject commit 7b357427a7f6578d3d959622fe1a6c1e402d1e5d