Add link to information about EOL to security announcement

Problem/Motivation

There is no notification in the Drupal Admin UI about D7 EOL.

Proposed resolution

Add link to page on Drupal.org with information about D7 EOL plan.
Provide information about Drupal 7 EOL date in the message on any Drupal website that notifies site admin that security update is available .

Remaining tasks

Add text changes in update module.

User interface changes