I am having CUCM 184.108.40.20600-6 and Phones are 8841 model IP Phone. Will this phone model supports Configuration File Encryption in this CUCM version?
Yes it will,
no need to put cluster in mixed mode
Yes, configuration file encryption is possible.
You may check the Security Guide (This is for 11.5) https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/security/11_5_1/secugd/CUCM_BK_SEE2CFE1_00_cucm-security-guide-1151/CUCM_BK_SEE2CFE1_00_cucm-security-guide-1151_chapter_01011.html
But I cannot see 8841 Phone in the document for CUCM 11.5. But I can see it in Security Guide for CUCM 12.0. So, Should I upgrade the call manager version?
Have you not looked at your server to find out if the option is included for that model or not??