feat: Make the arrow_drop_down icons in the QuillToolbar the same size for all MenuAnchor buttons

[d-ukhanov]

Description

This PR unifies the size of the arrow_drop_down icon for MenuAnchor buttons in the QuillToolbar as suggested in #1799

Related Issues

Feat #1799

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.