Corrects possible initializer exception in TeiidServerVersion

* Initialising the static DEFAULT_TEIID_SERVER using the enum Version can
  cause an InitialiserException due to the circularity of initialising the
  TeiidServerVersion class and Version enum while assigning the Version to
  a static.

* Moves the Default Version to the Version enum and refactors other classes
  accordingly.

* Corrects the Default Teiid Version to 8.7 from 8.6