Update git submodules

* Update docs/submodules/releng from branch 'master'
  to e32cfea0f1be6b86c0d528eb477b7265cae1a23e
  - Merge "Prune Docker images on lf-build5/6 to save space"
  - Prune Docker images on lf-build5/6 to save space

    This change also removed old builders which no longer exist.

    Change-Id: I233924ba2dd8e78ab6d73e0a13a785bb732cf1f4
    Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 14aa21a..e32cfea 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 14aa21ae04c08dbec43e66a01b0b8a6b88030b18
+Subproject commit e32cfea0f1be6b86c0d528eb477b7265cae1a23e