findmissing: Drop v5.6

Linux v5.6.y is EOL.

BUG=None
TEST=Run tools

Change-Id: I4d1767226bbee8e14a087a8c69ca618fa3261830
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2254301
Commit-Queue: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
diff --git a/contrib/findmissing/common.py b/contrib/findmissing/common.py
index 45592da..feb71c9 100755
--- a/contrib/findmissing/common.py
+++ b/contrib/findmissing/common.py
@@ -33,7 +33,7 @@
 
 # Order BRANCHES from oldest to newest
 CHROMEOS_BRANCHES = ['4.4', '4.14', '4.19', '5.4']
-STABLE_BRANCHES = ['4.4', '4.9', '4.14', '4.19', '5.4', '5.6', '5.7']
+STABLE_BRANCHES = ['4.4', '4.9', '4.14', '4.19', '5.4', '5.7']
 
 UPSTREAM_START_BRANCH = 'v%s' % CHROMEOS_BRANCHES[0]