Update Elasticsearch support policy after 8.0 release (elastic#1435)

Co-authored-by: Rick Boyd <[email protected]>

docs/migrate.rst

@@ -4,6 +4,14 @@ Migration Guide
 Migrating to Rally 2.4.0
 ------------------------
 
+Support for Elasticsearch < 6.8.0 has been dropped
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To benchmark Elasticsearch 6.x nodes, you need to upgrade to 6.8.x first.
+
+JSON for cli arguments that accept comma-separated values
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 Rally now accepts JSON files (ending in .json) or inline JSON strings for cli arguments that accept comma separated values like `--car`, `--telemetry` etc. The only allowed JSON is a plain array e.g. ``esrally race ... --car='["4g", "trial-license"]'``.
 
 Migrating to Rally 2.3.0

docs/versions.rst

@@ -10,7 +10,7 @@ However, Rally does not support Elasticsearch clusters using the OSS license.
 End-of-life Policy
 ==================
 
-The latest version of Rally allows to benchmark all currently supported versions of Elasticsearch. Once an `Elasticsearch version reaches end-of-life <https://www.elastic.co/support/eol>`_, Rally will support benchmarking the corresponding version for two more years. For example, Elasticsearch 1.7.x has been supported until 2017-01-16. Rally drops support for Elasticsearch 1.7.x two years after that date on 2019-01-16. Version support is dropped in the next Rally maintenance release after that date.
+The latest version of Rally allows to benchmark all currently supported versions of Elasticsearch. Once an `Elasticsearch version reaches end-of-life <https://www.elastic.co/support/eol>`_, Rally will support benchmarking its last minor version until a new major version comes out. For example, after the release of Elasticsearch 8.0.0, Rally dropped support for Elasticsearch < 6.8.0.
 
 Metrics store
 =============

esrally/min-es-version.txt

@@ -1 +1 @@
-6.0.0
+6.8.0