Replace print with stdout.writeln in StandardLogger

[kuhnroyal]

This PR replaces on instance on print with stdout.writeln in StandardLogger.
When having zones with custom print handling, the use of print here may lead to very unexpected outputs.
There is no other place where print is used so this is in line with all other logger methods.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.