CSPELL: Fix spelling for breaking variables and/or identifiers

Problem/Motivation

This task is a follow-up to 📌 GitlabCI: Fix CSPELL validation errors Needs review where an initial round of fixes for the CSPELL job on GitlabCI was done, but certain keywords/variable names or identifiers could not be changed without potentially breaking string translations or backward compatibility.

If these words need to be changed so they can be removed from the ignore list, an upgrade path would have to be provided and the changes would probably have to be part of a minor or major release.

Steps to reproduce

Execute locally:

cd web core
./node_modules/.bin/cspell -c .cspell.json --show-suggestions --show-context --no-progress ../modules/contrib/colorbox/

Remove the keywords to be modified from the ignore list, among which:

slideshowauto
slideshowspeed

# Config variables defined in module's schema file:
# Watchout: changing these would have a big impact and could break sites.
initialheight
initialwidth
maxheight
maxwidth
overlayclose
returnfocus

The word please in the file colorbox.install.

Proposed resolution

1 - Fix the wording of these keywords based on CSpell suggestions.

2 - Provide an upgrade path for all modified configuration variables, most likely a hook_update with instructions and/or documentation on any potentially breaking change, such as for string translations and form field IDs.