Fixed bad asciidoc link

Document thread-safety for ingest processors

With this commit we document that ingest processors need to be
thread-safe. Previously this could be inferred from reading the
source code but we got several user questions about this so
it is stated explicitly in the Javadocs of Processor now.

Build: Apply license section in poms only to elasticsearch artifacts (e…

…lastic#21757)

release [1.7.6]

Remove cluster update task when task times out (elastic#21578)

Fixes an issue where the cluster service does not remove an update task from its internal data structures that are used for batching cluster state updates.

Set vm.max_map_count on systemd package install

Our docs claim that we set vm.max_map_count automatically. This is not
quite the case. The story is that on SysV init we set vm.max_map_count
each time the service starts, which is good. On systemd, we create a
sysctl.d conf file that sets vm.map_max_count, but this is only
meaningful if the system is rebooted after package install. This commit
modifies the post-install script so that we run systemd-sysctl so that
the vm.max_map_count change occurs after package install without a
reboot.

Relates elastic#21507

Add correct Content-Length on HEAD requests

This commit fixes responses to HEAD requests so that the value of the
Content-Length is correct per the HTTP spec. Namely, the value of this
header should be equal to the Content-Length if the request were not a
HEAD request.

This commit also fixes a memory leak on HEAD requests to the main action
that arose from the bytes on a builder not being released due to them
being dropped on the floor to ensure that the response to the main
action did not have a body.

Relates elastic#21123

Updated release notes for 5.0.0-rc1

Added 2.4.1 release notes

Convert more docs to CONSOLE

* reference/api-conventions.asciidoc
* reference/redirects.asciidoc

Relates to elastic#18160