Change BTE version to 0.9.2.3 for DS-1857

We released version 0.9.2.3 of BTE that only fixes bugs and is compiled with java 1.6. This PR updates the dspace-api/pom.xml to this version of BTE.

It does fix the bug in DS-1857 and compiles/passes the tests of DSpace with java 1.6.

Thank you,
Panagiotis