[DOC] EAP7 repo symlinks are gonna be removed - update relevant doc parts

As we removed RHEL6 i386 as a supported platform with EAP7.2, there is a problem to continue to provide 'symlink repo' feature which gives customer possibility to be upgraded between minor versions without explicitely changing RPM repository configuraiton.

At least following guides have to be updated for EAP 7.2+ (provided links navigates to 7.1) so customers are properly informed and aware of this change:

1. Installation guide - RPM Installation - Choosing a repository section: there will be no 'current' repository for 7.2+, thus remove/reformulate that info from there
2. Patching and upgrading guide - Upgrading an RPM Installation: no 'current' repository is available for 7.2+, thus remove/reformulate that info from there.
3. Release Notes - we should add an info that 'current' RPM repositories are deprecated and will be discontinued. Customers who use 'current' RPM repository and want to upgrade to 7.2 need to update their repository configuration on the machines as described in patching guide.

If you know about any other places, feel free to update this list.