REL: run release notes for v1.21.0

[tangkong]

Description

Run release notes for hutch-python

Motivation and Context

🏷️ ⏲️

How Has This Been Tested?

N/A

Where Has This Been Documented?

This is Documentation

Pre-merge checklist

• Code works interactively (a real hutch config file can be loaded)
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

Looks good to me!

[janeliu-slac]

Everything looks good.