boost: update to 1.73

Upstream yocto has moved to 1.73 so get CI in line with that

Change-Id: Idbe71d4f2cf579375426f062bc084bea1b6567cf
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index f8e8889..25a3cfd 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -91,7 +91,7 @@
 # A list of package versions we are building
 # Start off by listing the stating versions of third-party sources
 declare -A PKG_REV=(
-  [boost]=1.71.0
+  [boost]=1.73.0
   [cereal]=v1.2.2
   [catch2]=v2.11.1
   [CLI11]=v1.9.0