chore(version_1): release 1.21.3

[release-please[bot]]

:robot: I have created a release beep boop

1.21.3 (2024-04-19)

Bug Fixes

• Correctly log Exceptions (#268) (713969b)
• deps: Update golang.org/x/net and @google-cloud/spanner to fix vulnerabilities. (#272) (f4e1158)

---

This PR was generated with Release Please. See documentation.

[release-please[bot]]

:robot: Release is at https://github.com/cloudspannerecosystem/autoscaler/releases/tag/v1.21.3 :sunflower: