Updates failing on CloudLinux server: "cannot install the best update candidate for package"

Symptoms

When upcp or yum runs to update the system packages, an error similar to the following will occur.

Problem: problem with installed package libxml2-2.9.7-13.el8_6.1.x86_64
- cannot install the best update candidate for package libxml2-2.9.7-13.el8_6.1.x86_64
- package libxslt-devel-1.1.32-6.el8.x86_64 requires pkgconfig(libxml-2.0), but none of the providers can be installed
- libxml2-devel-2.9.7-13.el8_6.1.i686 has inferior architecture
- libxml2-devel-2.9.7-13.el8.i686 has inferior architecture
- package libxml2-devel-2.9.7-13.el8_6.1.x86_64 requires libxml2(x86-64) = 2.9.7-13.el8_6.1, but none of the providers can be installed
- package libxml2-devel-2.9.7-13.el8.x86_64 requires libxml2(x86-64) = 2.9.7-13.el8, but none of the providers can be installed
- cannot install both libxml2-2.9.7-15.el8.x86_64 and libxml2-2.9.7-13.el8_6.1.x86_64
- cannot install both libxml2-2.9.7-15.el8.x86_64 and libxml2-2.9.7-13.el8.x86_64
- package alt-libxml2-devel-2.9.4-3.el8.x86_64 requires alt-libxml2 = 2.9.4-3.el8, but none of the providers can be installed
- cannot install the best update candidate for package alt-libxml2-2.10.2-1.el8.x86_64
- cannot install both alt-libxml2-2.9.4-3.el8.x86_64 and alt-libxml2-2.10.2-1.el8.x86_64
- cannot install the best update candidate for package libxslt-devel-1.1.32-6.el8.x86_64
- problem with installed package libxslt-devel-1.1.32-6.el8.x86_64

Description

The error commonly occurs when the cl-ea4-testing or other CloudLinux testing repository are enabled.

Workaround

1. Access the server's command line as the 'root' user via SSH or "Terminal" in WHM.
2. List the enabled testing repositories.

   [root@server ~]cPs# yum repolist|grep testing
   cl-ea4-testing CloudLinux 8 EasyApache4 - testing

3. Disable any testing repositories.

   yum-config-manager --disable $testingrepo

   Please note that "$testingrepo" must be replaced with the name of the testing repository found with the previous command.
4. Use yum to update the packages.

   yum update