- Improved FormatMap: Improves performance by about 4x, or nearly 2x in the worst case ([#112661](https://github.com/kubernetes/kubernetes/pull/112661), [@aimuz](https://github.com/aimuz)) [SIG Node]
- Improved misleading message, in case of no metrics received for the `HPA` controlled pods. ([#114740](https://github.com/kubernetes/kubernetes/pull/114740), [@kushagra98](https://github.com/kushagra98))

- Base-images: Update to debian-iptables:buster-v1.4.0
  - Uses iptables 1.8.5
  - base-images: Update to debian-base:buster-v1.3.0
  - cluster/images/etcd: Build etcd:3.4.13-2 image
    - Uses debian-base:buster-v1.3.0 ([#98401](https://github.com/kubernetes/kubernetes/pull/98401), [@pacoxu](https://github.com/pacoxu)) [SIG Testing]

## Changes by Kind

### Feature

- Kubernetes is now built with go 1.21.9
  - update debian-base/set-cap to bookworm-v1.0.2 ([#124198](https://github.com/kubernetes/kubernetes/pull/124198), [@cpanato](https://github.com/cpanato)) [SIG API Machinery, Architecture, Release and Testing]

### Bug or Regression

## Changelog since v1.8.14

### Action Required

* ACTION REQUIRED: Kubernetes JSON deserializer is now case-sensitive to restore compatibility with pre-1.8 servers. If your config files contains fields with wrong case, the config files will be now invalid. ([#65410](https://github.com/kubernetes/kubernetes/pull/65410), [@nikhita](https://github.com/nikhita))

### Other notable changes

- Ignore the case when comparing azure tags in service annotation. ([#104705](https://github.com/kubernetes/kubernetes/pull/104705), [@nilo19](https://github.com/nilo19))
- Ignore the case when updating Azure tags. ([#104593](https://github.com/kubernetes/kubernetes/pull/104593), [@nilo19](https://github.com/nilo19))

- The Go API for logging configuration in `k8s.io/component-base` was moved to `k8s.io/component-base/logs/api/v1`. The configuration file format and command line flags are the same as before. ([#105797](https://github.com/kubernetes/kubernetes/pull/105797), [@pohly](https://github.com/pohly))

- Fixed an issue where kubelet does not set case-insensitive headers for http probes. (#117182, @dddddai) ([#117331](https://github.com/kubernetes/kubernetes/pull/117331), [@dddddai](https://github.com/dddddai)) [SIG API Machinery, Apps and Node]

- Base images: Updated to
  - debian-base:buster-v1.6.0
  - debian-iptables:buster-v1.6.0 ([#100976](https://github.com/kubernetes/kubernetes/pull/100976), [@jindijamie](https://github.com/jindijamie))

   ([#123343](https://github.com/kubernetes/kubernetes/pull/123343), [@haircommander](https://github.com/haircommander))
- Updated `distroless-iptables` to `v0.5.0`, debian-base to `bookworm-v1.0.1`, and setcap to `bookworm-v1.0.1`.
   ([#123170](https://github.com/kubernetes/kubernetes/pull/123170), [@cpanato](https://github.com/cpanato))
- `NewVolumeManagerReconstruction` feature is now GA.

- Component-base: make the validation logic about LeaderElectionConfiguration consistent between component-base and client-go ([#111758](https://github.com/kubernetes/kubernetes/pull/111758), [@SataQiu](https://github.com/SataQiu)) [SIG API Machinery and Scheduling]