Unlist timeline subheadings

Signed-off-by: Ben Sherman <bentshermann@gmail.com>

Author: bentsherman

docs/migrating-plugin-registry.md

@@ -28,21 +28,21 @@ The [legacy plugin index](https://github.com/nextflow-io/plugins) will be deprec
 The following timeline is tentative and subject to modification.
 :::
 
-### Nextflow 25.04
+<h3>Nextflow 25.04</h3>
 
 The Nextflow plugin registry is available as a private beta. Nextflow 25.04 can use the Nextflow plugin registry as an opt-in feature. The Nextflow plugin registry will be automatically kept up-to-date with the [legacy plugin index](https://github.com/nextflow-io/plugins).
 
 During this time, plugin developers are encouraged to experiment with the Gradle plugin and plugin registry.
 
-### Nextflow 25.10
+<h3>Nextflow 25.10</h3>
 
 The Nextflow plugin registry will be generally available. Nextflow 25.10 will use the plugin registry by default. The legacy plugin index will be **closed to new pull requests**.
 
 Developers will be required to publish to the Nextflow plugin registry. To ensure continued support for older versions of Nextflow, the legacy plugin index will be automatically kept up-to-date with the Nextflow plugin registry.
 
-### Nextflow 26.04
+<h3>Nextflow 26.04</h3>
 
-Nextflow 25.10 will only be able to use the Nextflow plugin registry.
+Nextflow 26.04 will only be able to use the Nextflow plugin registry.
 
 At some point in the future, the legacy plugin index will be **frozen** -- it will no longer receives updates from the Nextflow plugin registry. To ensure continued support for older versions of Nextflow, the legacy plugin index will remain available indefinitely.