admin_guide/encrypting_data: add note about required etcd version

[php-coder]

To reduce confusion of the users who are still on etcd2 but would like to try data encryption, I added a note that this feature only works on etcd3.

The identical changes have been already made for Kubernetes: kubernetes/website#5521

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1502767

PTAL @smarterclayton @openshift/team-documentation
CC @simo5

[php-coder]

BTW do we need to also warn users that data encryption is in alpha state?

[mburke5678]

@php-coder LGTM. Probably should add an alpha note.
Other places we use: This is an alpha feature and may change in future

[php-coder]

@mburke5678 Done. PTAL.

[mburke5678]

@php-coder LGTM. Ready to merge? This is for 3.6 and forward?

[php-coder]

Ready to merge?

Yes, please.

This is for 3.6 and forward?

Yes, exactly.

[mburke5678]

[rev_history]
|xref:../admin_guide/encrypting_data.adoc#admin-guide-encrypting-data-at-datastore[Encrypting Data at Datastore Layer]
|Added a note that etcd v3 or later is required in order to use data encyption.
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[php-coder]

@mburke5678 Thanks!

I'm wondering why it has only enterprise tags, while it also available in origin, for example?

[mburke5678]

@php-coder

while it also available in origin

Origin is based off the master branch. As soon as a PR is merged to master it will go to Origin.