Werk #17292: check_httpv2: Body checking: Fixes for inverted regular expressions

The new active check for HTTP endpoints offers the option to search the response body for strings. This search can be configured to either use a fixed string or a regular expression. In the latter case, there is the additional option to invert the matching: Report WARNING if the expression matches and OK if not. This werk fixes two issues related to this option:

1. In the user interface, the inversion option was incorrectly labelled with "CRITICAL" instead of "WARNING" in case the expression matches.

2. In the service output, if inversion was activated, a not-matched expression was labelled with "matched" and a matched expression with "not matched". Note that the actual service state (WARNING if matched, OK otherwise) was correct.

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