Update changelog and version after v2.21.8

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 See the [releases page](https://github.com/github/codeql-action/releases) for the relevant changes to the CodeQL CLI and language packs.
 
+## [UNRELEASED]
+
+No user facing changes.
+
 ## 2.21.8 - 19 Sep 2023
 
 - Add a deprecation warning for customers using CodeQL version 2.10.4 and earlier. These versions of CodeQL were discontinued on 12 September 2023 alongside GitHub Enterprise Server 3.6, and will be unsupported by the next minor release of the CodeQL Action. [#1884](https://github.com/github/codeql-action/pull/1884)

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "codeql",
-  "version": "2.21.8",
+  "version": "2.21.9",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "codeql",
-      "version": "2.21.8",
+      "version": "2.21.9",
       "license": "MIT",
       "dependencies": {
         "@actions/artifact": "^1.1.2",

package.json

@@ -1,6 +1,6 @@
 {
   "name": "codeql",
-  "version": "2.21.8",
+  "version": "2.21.9",
   "private": true,
   "description": "CodeQL action",
   "scripts": {