deps: Manage mutation-testing-elements with renovate

[hanahbaker1]

Adding custom manager for libman in renovate config file
fixes #2862

[rouke-broersma]

@hanahbaker1 apparently for renovate to validate the change the branch must be named renovate/reconfigure. I've copied your change to a new branch with that name and the validation seems to be failing. I had expected some kind of error message from renovate that explains what's wrong but so far I haven't found any. See: #2910

Seems like something in the configuration is incorrect.

[rouke-broersma]

Ok I ran the validator locally and this is the output:

> RENOVATE_CONFIG_FILE=renovate.json

> npx -p renovate renovate-config-validator

 INFO: Validating renovate.json
ERROR: Found errors in configuration
       "file": "renovate.json",
       "errors": [
         {
           "topic": "Configuration Error",
           "message": "Custom Manager contains disallowed fields: depName"
         },
         {
           "topic": "Configuration Error",
           "message": "Invalid configuration option: customManagers[0].depName"
         }
       ]

[rouke-broersma]

The config is not quite valid. You can validate the config using the following commands on linux:

> cd stryker-net/.github
> RENOVATE_CONFIG_FILE=renovate.json
> npx -p renovate renovate-config-validator

Let me know if you need any help with that