This commit changes the sysprop for overriding the branch bwc builds use
to be branch specific. There are 3 different bwc branches built, but all
of them currently read the exact same sysprop. For example, with this change
and current branches, you can now specify eg `-Dtests.bwc.refspec.6.x=my_6x`
and it will build only next-minor-snapshot with that branch, while
next-bugfix-snapshot will continue to use 5.6.

* master: (80 commits)
  Remove HTTP max content length leniency (elastic#29337)
  Begin moving XContent to a separate lib/artifact (elastic#29300)
  Java versions for ci (elastic#29320)
  Minor cleanup in the InternalEngine (elastic#29241)
  Clarify expectations of false positives/negatives (elastic#27964)
  Update docs on vertex ordering (elastic#27963)
  Revert "REST high-level client: add support for Indices Update Settings API (elastic#28892)" (elastic#29323)
  [test] remove Streamable serde assertions (elastic#29307)
  Improve query string docs (elastic#28882)
  fix query string example for boolean query (elastic#28881)
  Resolve unchecked cast warnings introduced with elastic#28892
  REST high-level client: add support for Indices Update Settings API (elastic#28892)
  Search: Validate script query is run with a single script (elastic#29304)
  [DOCS] Added info on WGS-84. Closes issue elastic#3590 (elastic#29305)
  Increase timeout on Netty client latch for tests
  Build: Use branch specific refspec sysprop for bwc builds (elastic#29299)
  TEST: trim unsafe commits before opening engine
  Move trimming unsafe commits from engine ctor to store (elastic#29260)
  Fix incorrect geohash for lat 90, lon 180 (elastic#29256)
  Do not load global state when deleting a snapshot (elastic#29278)
  ...