Bulk update 8.6.x & 8.7.x Drupal core issues for 8.8.x release cycle opening

Sub-issue of #2631820: [policy, then infra] Bulk update core issues for relevant minor versions.

There are two bulk updates that need to happen each minor:

1. After branching 8.8.x
   In advance of Drupal 8.7.0-alpha1 (to be released week of March 11), open issues need to be updated that are now listed for 8.7.x to be under 8.8.x. This should ideally happen on the Thursday or Friday before the alpha release, around March 8. Message:

   <a href="https://groups.drupal.org/node/534845">Drupal 8.7.0-alpha1</a> will be released the week of March 11, 2019, which means new developments and disruptive changes should now be targeted against the 8.8.x-dev branch. For more information see the <a href="/core/release-cycle-overview#minor">Drupal 8 minor version schedule</a> and the <a href="/core/d8-allowed-changes">Allowed changes during the Drupal 8 release cycle</a>.

2. Separately, after the last planned bugfix release of 8.6.x, we create a separate bulk update like #2900468: Bulk update 8.3.x Drupal core issues to 8.4.x after 8.3.6 with a different message. Open issues need to be updated that are now listed for 8.6.x to be under 8.7.x:
   

   <a href="/project/drupal/releases/8.6.TODO">Drupal 8.6.TODO</a> was released on TODO and is the final full bugfix release for the Drupal 8.6.x series. Drupal 8.6.x will not receive any further development aside from security fixes. Sites should prepare to update to 8.7.0 on TODO. (<a href="/project/drupal/releases/8.7.0-rc1">Drupal 8.7.0-rc1</a> is available for testing.)
   
   Bug reports should be targeted against the 8.7.x-dev branch from now on, and new development or disruptive changes should be targeted against the 8.8.x-dev branch. For more information see the <a href="/core/release-cycle-overview#minor">Drupal 8 minor version schedule</a> and the <a href="/core/d8-allowed-changes">Allowed changes during the Drupal 8 release cycle</a>.

A script used to do this is available in #2631820-25: [policy, then infra] Bulk update core issues for relevant minor versions.

This is a successor issue to #2985640: Bulk update 8.6.x Drupal core issues to 8.7.x in advance of alpha1.