Update GitHub actions/checkout to version 6

Fix deprecation warning for Node.js 20.

Change-Id: I6c2a83e9fe54e7c7d6db9aabdd16aaf5b1bac7a0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/96587
Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Lily Chen <chlily@google.com>
Commit-Queue: Lily Chen <chlily@google.com>

.github/workflows/branch-time.yml

diff --git a/.github/workflows/branch-time.yml b/.github/workflows/branch-time.yml
index 49f74d7..20d9eb2 100644
--- a/.github/workflows/branch-time.yml
+++ b/.github/workflows/branch-time.yml
@@ -32,7 +32,7 @@
     permissions: read-all
     steps:
       - name: Checkout GitHub Repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           ref: 'main'
           fetch-depth:  0 # Required to accurately count commits and find ancestors