DEPENDENCIES: bump required yml version

Even if the engine could live with 2.7.0, the latest libpEpTransport needs 2.7.1.
Let us use it.  Also add libpEpTransport as a dependency, even if the automatic
gitlab build system does not support it yet.

DEPENDENCIES

@@ -1,4 +1,5 @@
 # 1st Party Dependencies
 ## Prefer git tags instead of SHA hashes when possible.
 sequoia=openpgp/v1.3.0
-yml2=2.7.0
+yml2=2.7.1
+libpEpTransport=8acf7525f31ec6c4ae3f36cc42cf297ddfbb6b35