Upgrade example/echo to Bazel 7.0.0

Bug: 318850523
Change-Id: I987e91e7ae7132bb8c357d0b08f46c4668bc3005
Reviewed-on: https://pigweed-review.googlesource.com/c/example/echo/+/187510
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Reviewed-by: Austin Foxley <afoxley@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/.bazelversion b/.bazelversion
index 91e4a9f..66ce77b 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-6.3.2
+7.0.0