Ensure parsed `user_agents` is indexed.

[simitt]

Keep track of elastic/elasticsearch#37329.

If the changes get merged in time for 7.0 ensure we change the user_agent_parsed (introduced in #1783) to user_agent and that fields get indexed.

Otherwise ensure that in time for 7.0 we either

• index the current user_agent_parsed fields or
• overwrite ECS definitions from beats and move and index the information under user_agent with the downside that it is not ECS compliant.

[simitt]

PR in ES: elastic/elasticsearch#37727