chore: update the version number to 2.1.0-SNAPSHOT

[delei]

Purpose of the pull request

Closed: #859

What's changed?

1. update flatten-maven-plugin configuration for fesod-bom module.
   https://repository.apache.org/content/groups/snapshots/org/apache/fesod/fesod-bom/2.1.0-SNAPSHOT/fesod-bom-2.1.0-20260304.154925-3.pom

2. update the version number to 2.1.0-SNAPSHOT and publish the snapshot to the Apache Maven Central Repository.
   https://repository.apache.org/content/groups/snapshots/org/apache/fesod/

3. introduce and configure versions-maven-plugin , when releasing a new version, it is convenient to batch modification the version numbers of the parent project and all its sub-modules.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.