have towncrier use h2 header for each release note

Author: d-v-b

pyproject.toml

@@ -445,6 +445,7 @@ checks = [
 directory = 'changes'
 filename = "docs/release-notes.md"
 underlines = ["", "", ""]
+title_format = "## {version} ({project_date})"
 issue_format = "[#{issue}](https://github.com/zarr-developers/zarr-python/issues/{issue})"
 start_string = "<!-- towncrier release notes start -->\n"