Update version to 2.1.0 in REL_2_STABLE

[tuhaihe]

closes: #ISSUE_Number

---

Change logs

Describe your change clearly, including what problem is being solved or what document is being added or updated.

Contributor's checklist

Here are some reminders before you submit your pull request:

• Make sure that your Pull Request has a clear title and commit message. You can take the Git commit template as a reference.
• Learn the code contribution and doc contribution guides for better collaboration.
• Make sure that CICD workflow is successful.
• List your communications in the GitHub Issues or Discussions (if has or needed).
• Feel free to ask for the cloudberry committers or other people to help review and approve.

[tuhaihe]

Hi @ostinru do you know this file https://github.com/apache/cloudberry-pxf/blob/main/api_version needs updates to keep align with the file version ?

[MisterRaindrop]

api_version FDW client (C-side) and the PXF server (Java-side) communicate via HTTP, and the URL includes this API version:

http://<pxf_host>:/pxf/v16/read

Its purpose is to ensure protocol compatibility between the client and server. If incompatible changes are made to the API, the version number will be incremented, allowing the server to reject requests from older clients that do not match.