Docs-3076 Remove GELI instructions from CORE Storage Encryption Article

[micjohnson777]

Removed detailed GELI migration instructions from the article, added information about 13.0 release and need to revert back to a 12.0 release to migrate a GELI pool to a ZFS pool. Fixed a few typos and sentence structure in a few places.

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[micjohnson777]

Need to add a link to the 12.0 article content with instructions to migrate a GELI pool. Procedure only exists in the article so no archive link exists yet.

[BonnieF]

Is this one Supposed to be Merged into Master or in 13.0 Documentation?

[BonnieF]

We need to add a link for the GELI content from archived copy of 12.0 content. Tim is going to help Mic set up a dummy link so that, when the archive is created, it will work (when we merge into 13.0)