Rename MD5_HASH to MD5_HEX

[open-gdsn]

Since that is what it actually is... ;-)

https://github.com/holodeck-b2b/bdxr-common/blob/821cd50d99b2f144bcb31b23b59b4c12e25a7b52/generic-client/src/test/java/org/holodeckb2b/bdxr/sml/PEPPOLHostNameGeneratorTest.java#L34

[Holodeck B2B]

It actually is the Hex representation of the hash, so the most complete name here would even be MD5_HASH_HEX :-)
But as its just in a test class and the meaning still clear this change is also low prio.