Update CHANGELOG.md

[jasonpcarroll]

Update CHANGELOG.md with all PRs merged since v2.1.0 for the v3.1.2 release.

Changes include:

• Add write permissions to doxygen generation workflow on main #200 - Add write permissions to doxygen generation workflow
• Fix MISRA violations. #199 - Fix MISRA violations
• Add CMakeLists.txt for coreHTTP library #197 - Add CMakeLists.txt for coreHTTP library
• Update manifest to list CMock as a test dependency. #196 - Update manifest to list CMock as a test dependency
• Remove version numbers from source files and add library version macro. #195 - Remove version numbers from source files
• Return HTTPSuccess when HTTP_RESPONSE_DO_NOT_PARSE_BODY_FLAG is set #194 - Return HTTPSuccess when HTTP_RESPONSE_DO_NOT_PARSE_BODY_FLAG is set
• Remove formatting bot workflow #191 - Remove formatting bot workflow
• Fix comment in config defaults header file #189 - Fix comment in config defaults header file
• Follow Security Guide to update YAML files. #188 - Follow Security Guide to update YAML files
• Update GitHub Actions workflows as per security guidelines #187 - Update GitHub Actions workflows as per security guidelines
• Replace Synopsys link with blackduck one to solve link error. #186 - Replace Synopsys link with blackduck one
• Adjust proof tooling to support CBMC v6 #184 - Adjust proof tooling to support CBMC v6
• Update LTS 202406 information #183 - Update LTS 202406 information
• Bump CBMC version to 5.95.1 #181 - Bump CBMC version to 5.95.1