Improve text on database update page

Problem/Motivation

From comment [#2099099-52

Aside that it might be probably a good idea to rework the micro copy of the Drupal database update page in general in a follow up issue. For example it isn't clear at all what does Update your files (as described in the handbook page linked above). in step 4 actually refer to?

The upgrading handbook link it points to doesn't contain any clues in regards of "Update your files". If you search for update file or update your files you have no match - the linked https://www.drupal.org/updating-and-upgrading-drupal-core → was last updated on the 5th of December 2017. It might make sense to review if that page might need an update as well. At least as a resource in the context of the provided link on the Drupal database update it is not that helpful at the moment.

On a side note, directional instructions on the Drupal database update page like "steps above" pointing to the handbook page should be avoided. It is not advised to utilize any directional instructions/language that requires the reader to see the layout or design of the page (for example https://styleguide.mailchimp.com/writing-for-accessibility/).

Steps to reproduce

Go to /update.php

Proposed resolution

Update the text and links

Remaining tasks

MR
Review

User interface changes

API changes

Data model changes

Release notes snippet