Update EULA page re installer #105

[phillxnet]

Drop "Rockstor Leap 15.4" nomenclature in favour of the more representative 'Rockstor "Built on openSUSE" Leap 15.6'.

These changes are intended to normalise our reference re our base OS, and to hopefully better adhere to the upstream guidelines as per:

• https://en.opensuse.org/openSUSE:Trademark_guidelines

N.B. in-file formatting, within EULA section, intentionally follows that of the original, as presented to the end-user during install.

Includes

• Copyright date update.
• Specify 'Stable Updates' as Subscription type: we have others.
• Acknowledge version differences.
• Remove now redundant references.
• Minor EULA section introduction/pre-able rewording.
• Preserve underlying references to "The openSUSE Project".
• Preserve "SUSE" & "openSUSE" trademarks attribution: SUSE LLC.
• Preserve upstreams legal reference given our "Built on ..." status.

[phillxnet]

A dependency of this PR is the partner PR in https://github.com/rockstor/rockstor-installer to which this PR, in-turn references a modified file there, and the text changes, EULA wise, are also as a result of the proposed changes specifically in:

• "Update copyright & EULA sed edits ...": https://github.com/rockstor/rockstor-installer/pull/188

As such that issue should be merged first.

[phillxnet]

Once we are just a little closer to releasing our newly rebuild installers I'll merge and get this published. Given it reflects what those rebuild installers will show, not what is currently shown by our existing installers. Bit of a chicken and egg one this but shouldn't be too much longer before we can build and publish the newer installers.