Remove unused gcov: true for codecov-action@v4

[hugovk]

The gcov argument has been removed codecov/codecov-action@v4 and has been replaced by an optional plugin that includes gcov by default:

Input	Description	Required
plugin	plugins to run. Options: xcode, gcov, pycoverage. The default behavior runs them all.

https://github.com/codecov/codecov-action#arguments

This will remove some warnings like:

Unexpected input(s) 'gcov', valid inputs are ['token', 'codecov_yml_path', 'commit_parent', 'directory', 'disable_file_fixes', 'disable_search', 'disable_safe_directory', 'dry_run', 'env_vars', 'exclude', 'fail_ci_if_error', 'file', 'files', 'flags', 'git_service', 'handle_no_reports_found', 'job_code', 'name', 'network_filter', 'network_prefix', 'os', 'override_branch', 'override_build', 'override_build_url', 'override_commit', 'override_pr', 'plugin', 'plugins', 'report_code', 'root_dir', 'slug', 'url', 'use_legacy_upload_endpoint', 'use_oidc', 'verbose', 'version', 'working-directory']

https://github.com/python-pillow/Pillow/actions/runs/11641379867
https://github.com/python-pillow/Pillow/actions/runs/11641379864