Remove Stamen key from CFG file

[Gustry]

Amen

Related to #3914

Note, I didn't remove it from tests/units/classes/Project/Ressources, as I might break JS units, and there is still the code in the main code.

[3liz-bot]

The backport to release_3_6 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 0972e35bb... Remove Stamen key from CFG file
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

CONFLICT (modify/delete): tests/end2end/playwright/base-layers.spec.js deleted in HEAD and modified in 0972e35bb (Remove Stamen key from CFG file).  Version 0972e35bb (Remove Stamen key from CFG file) of tests/end2end/playwright/base-layers.spec.js left in tree.
Auto-merging tests/qgis-projects/tests/base_layers.qgs.cfg
CONFLICT (content): Merge conflict in tests/qgis-projects/tests/base_layers.qgs.cfg
Auto-merging tests/qgis-projects/tests/overview-3857.qgs.cfg

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release_3_6 release_3_6
# Navigate to the new working tree
cd .worktrees/backport-release_3_6
# Create a new branch
git switch --create backport-3987-to-release_3_6
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 0972e35bbc044ea1e8f76305010cd9089cf04374
# Push it to GitHub
git push --set-upstream origin backport-3987-to-release_3_6
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release_3_6

Then, create a pull request where the base branch is release_3_6 and the compare/head branch is backport-3987-to-release_3_6.