This commit overhauls the documentation of discovery and cluster coordination,
removing mention of the Zen Discovery module and replacing it with docs for the
new cluster coordination mechanism introduced in 7.0.
Relates #32006
Tweak the upgrade instructions for moving from pre-6.3-with-x-pack to
post-6.3-default distribution. Specifically, you have to remove the
x-pack plugin before upgrading because 6.4 doesn't understand how to
remove it.
Relates to #34307
Since we disable allocation using persistent settings, we should be consistent and remove
the setting from the persistent storage. Otherwise an accidental restart will leed for shards
not being allocated.
Relates to https://github.com/elastic/elasticsearch/pull/28757
Lisa Cawley
debadair
David Turner
Nik Everett
Boaz Leskes