Forward-port missing v2.15.1 upgrading docs to master branch

[julianbrost]

During the preparation of the v2.16.2 (and more) releases, I noticed that older support branches contained upgrading docs that are missing from the master branch.

In particular, the Upgrading to v2.15.1, v2.14.7, or v2.13.13 section is missing in both the support/2.16 branch (which is used to render https://icinga.com/docs/icinga-2/latest/doc/16-upgrading-icinga-2/), and the master branch (which will be used to create future support/* branches from).

This PR forward-ports the changes to that everything is on master, so that it's not lost. It's also supposed to be backported so that it appears on https://icinga.com/docs/icinga-2/latest/doc/16-upgrading-icinga-2/ again (where it was lost with the v2.16.0 release).

Quick and dirty shell script to verify what's missing (note that "Upgrading to v2.13.13" and "Upgrading to v2.14.7" are the same as added here, just with a different heading, and "Upgrading to v2.16.2, v2.15.4, or v2.14.9" will be added by #10909):

for branch in origin/support/2.{8..16}; do
  git show "$branch":doc/16-upgrading-icinga-2.md | grep '^#.*Upgrading to'
done | sort -u | while read -r heading; do
  grep -Fe "$heading" doc/16-upgrading-icinga-2.md || echo "MISSING: $heading"
done

[backbot-ci]

Successfully created backport PR for support/2.16:

• [Backport support/2.16] Add missing v2.15.1 upgrading docs to support/2.16 branch #10916