Update ESR Reference in the PHP requirements table and update php compatability statements as appropriate

Fixes #11

Changing link to point to existing documentation for installing l10n / localisation packages in Update index.md

I left the other part until we actually cut 5.40 due to the framing

CIVICRM-1707 Remove incorrect statement about open_basedir being deprecated. Safe mode was deprecated in PHP 5.3, so no longer relevant. Include minimum PHP configuration recommendations and links.

Update Requirements doc to mention that PHP7.4 is supported from MySQL 5.7 and also to clarify that all CMSes now supoort MySQL8 and add a note about installing on Cloud Service Providers such as AWS and Azure

This option was often set permissively in D8 project-templates - and it's
often set restrictively in D9 project-templates.

Adding the step generally shouldn't change anything for D8 admins -- and for
D9 admins it enables installation to proceed.

* This is predictably getting out of sync 'docs/drupal8'
* 'docs/drupal8' is getting updates to reference both 8+9

This is being migrated to the SysAdmin guide as it is no longer required
for new installations