Update Mechanism: Manual

This CL sets the Update Mechanism to Manual in README files.

Bug: 445311061
Change-Id: Ia600d99e21126d2130cd93330274ad136d5122aa
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/87647
Auto-Submit: Jordan Brown <rop@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

third_party/fiat/README.chromium

diff --git a/third_party/fiat/README.chromium b/third_party/fiat/README.chromium
index b2915c1..383024b 100644
--- a/third_party/fiat/README.chromium
+++ b/third_party/fiat/README.chromium
@@ -6,6 +6,7 @@
 License: Apache-2.0
 License File: LICENSE
 Security Critical: yes
+Update Mechanism: Manual
 Shipped: yes
 
 Description: