[Hacktoberfest] Improve benchmark UI, show the results of the last N releases. #771

Situation:

Benchmark test results are written as JSON to benchmarks.json which is saved in the gh-pages branch of the repo. The JSON used as a source for the chart UI.

Results as a chart can be found: https://commercetools.github.io/commercetools-sync-java/benchmarks/

Complication:

The current benchmark chart has more than 20 items and increasing on every release,
Also in the graph, we are pointing to the hash of the commit, but it’s not easy to reason and compare it based on the releases.

Resolution:
Show the results of the last N releases (N might be sth easy to reason from the chart, let's say 5) and backup the history to another file. For the label, try showing the release number instead of commit hash.

CONTRIBUTING, CODE_OF_CONDUCT