Fix deadlock when rendering annotation layers

[nyalldawson]

Description

Also potentially a cause of a lot of other deadlocks!

It's not safe to use a static QRegularExpression, these need to be thread_local instead. Otherwise we can hit a deadlock in Qt internals when accessing the same QRegularExpression across multiple threads.

AI tool usage

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[github-actions]

🍎 MacOS Qt6 builds

Download MacOS Qt6 builds of this PR for testing.
This app is not notarized, run sudo xattr -d com.apple.quarantine /Applications/QGIS*.app to avoid the warning
(Built from commit f21a443)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
To execute locally, unzip the downloaded zip file and run bin\qgis-bin.exe in the extracted directory.
You might be prompted by Windows Defender click "Run anyway"
(Built from commit f21a443)

[qgis-bot]

The backport to release-3_44 failed:

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

stderr

error: could not apply 18c3ce9454c... Fix deadlock when rendering annotation layers
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 set advice.mergeConflict false"

stdout

Auto-merging src/core/expression/qgsexpression.cpp
CONFLICT (content): Merge conflict in src/core/expression/qgsexpression.cpp

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_44 release-3_44
# Navigate to the new working tree
cd .worktrees/backport-release-3_44
# Create a new branch
git switch --create backport-66712-to-release-3_44
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 18c3ce9454c8a1568ad7d9655da625daf15068fe f21a443ae28522b1c77524461e4dc29f348dc5f6
# Push it to GitHub
git push --set-upstream origin backport-66712-to-release-3_44
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_44

Then, create a pull request where the base branch is release-3_44 and the compare/head branch is backport-66712-to-release-3_44.