Fix case-sensitivity of http header checks

[nyalldawson]

Since qt 6.8 header keys are forced to lowercase, so we need to adapt all checks accordingly

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 4caee87)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 4caee87)

[nyalldawson]

Unrelated test failure

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply 4caee87962c... Fix case-sensitivity of http header checks
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".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging src/providers/wfs/qgsbasenetworkrequest.cpp
Auto-merging tests/src/core/testqgsnetworkaccessmanager.cpp
Auto-merging tests/src/python/test_qgsblockingnetworkrequest.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgsblockingnetworkrequest.py

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-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-59619-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 4caee87962cbf99dd95e649ab863bab5bdfb4cac
# Push it to GitHub
git push --set-upstream origin backport-59619-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-59619-to-queued_ltr_backports.