Hi,
Enter this command in the elevated Command Prompt to stay on 23H2 for the time being. The update popup would disappear in a day or two.
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate" /v ProductVersion /t REG_SZ /d "Windows 11" /f && reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate" /v TargetReleaseVersion /t REG_DWORD /d 1 /f && reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate" /v TargetReleaseVersionInfo /t REG_SZ /d "23H2" /f