[ML] Remember v7 TODOs before beta1 is feature frozen

[elasticmachine]

Original comment by @droberts195:

We have a number of TODOs for version 7 in the ML code base.

We are not actioning these immediately, as the code divergence between the master and 6.x branches would make backporting more difficult. However, before 7.0.0-beta1 is released we should action the TODOs.

The current list is as follows:

plugin/src/main/java/org/elasticsearch/xpack/ml/action/OpenJobAction.java:        /** TODO Remove in 7.0.0 */
plugin/src/main/java/org/elasticsearch/xpack/ml/action/OpenJobAction.java:         * TODO: remove this member in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/action/OpenJobAction.java:            // TODO: remove leniency and reject the node if the attribute is null in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/action/OpenJobAction.java:            // TODO: remove leniency and reject the node if the attribute is null in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/datafeed/DatafeedConfig.java:            // TODO this is to read former _source field. Remove in v7.0.0
plugin/src/main/java/org/elasticsearch/xpack/ml/datafeed/DatafeedUpdate.java:            // TODO for former _source param - remove in v7.0.0
plugin/src/main/java/org/elasticsearch/xpack/ml/datafeed/DatafeedUpdate.java:            // TODO for former _source param - remove in v7.0.0
plugin/src/main/java/org/elasticsearch/xpack/ml/job/persistence/JobDataDeleter.java:        // TODO: remove in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/job/persistence/JobDataDeleter.java:            // TODO: change docDeleteListener to listener in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/job/persistence/JobStorageDeletionTask.java:                // TODO: remove in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/job/persistence/JobStorageDeletionTask.java:                // TODO: remove in 7.0
plugin/src/main/java/org/elasticsearch/xpack/ml/job/process/autodetect/AutodetectProcessManager.java:    // TODO: Remove the deprecated setting in 7.0 and move the default value to the replacement setting
plugin/src/main/java/org/elasticsearch/xpack/ml/MachineLearning.java:            // TODO: the simple true/false flag will not be required once all supported versions have the number - consider removing in 7.0

At the time someone works on this issue they can find the up-to-date list using:

find plugin/src/main/java/org/elasticsearch/xpack/ml -name '*.java' | xargs grep 'TODO.*7'

Also remove steps 1 and 2 from TransportOpenJobAction::masterOperation which update the version 5.4 mappings of the results and state indices