Assertions severity off is considered invalid

[adamaltman]

Describe the bug

The value off isn't accepted but it is described as an acceptable severity level.

• Invalid severity level; accepted values: error or warn.

It also results in a hard (not elegant) crash.

To Reproduce Steps to reproduce the behavior:

1. Given this .redocly.yaml file with

       assert/properties-include-examples:
         subject: SchemaProperties
         property: example
         defined: true
         nonEmpty: true
         severity: off
   

2. See error

Expected behavior

It should work with off. However, if there is a value someone didn't expect, then don't do a hard crash.

Logs

  - Invalid severity level; accepted values: error or warn.

/Users/adam/.nvm/versions/node/v16.10.0/lib/node_modules/@redocly/cli/node_modules/@redocly/openapi-core/lib/format/format.js:148
            throw new Error(ERROR_MESSAGE.INVALID_SEVERITY_LEVEL);
                  ^

Error: Invalid severity level; accepted values: error or warn
    at getBgColor (/Users/adam/.nvm/versions/node/v16.10.0/lib/node_modules/@redocly/cli/node_modules/@redocly/openapi-core/lib/format/format.js:148:19)
    at formatCodeframe (/Users/adam/.nvm/versions/node/v16.10.0/lib/node_modules/@redocly/cli/node_modules/@redocly/openapi-core/lib/format/format.js:153:25)
    at Object.formatProblems (/Users/adam/.nvm/versions/node/v16.10.0/lib/node_modules/@redocly/cli/node_modules/@redocly/openapi-core/lib/format/format.js:71:41)
    at Object.<anonymous> (/Users/adam/.nvm/versions/node/v16.10.0/lib/node_modules/@redocly/cli/lib/commands/lint.js:52:36)
    at Generator.next (<anonymous>)
    at fulfilled (/Users/adam/.nvm/versions/node/v16.10.0/lib/node_modules/@redocly/cli/lib/commands/lint.js:5:58)

OpenAPI definition If applicable, add an OpenAPI definition and .redocly.yaml configuration file that helps reproduce the problem. At a minimum, please state the specification version(s) you're using (e.g. 2.0, 3.0, 3.1).

Redocly Version(s) What version of Redocly CLI are you using?

Node.js Version(s) What version of node.js are you using?

Additional context Add any other context about the problem here.

[adamaltman]

Raising priority so that docs can be simplified.