all filter strings to lower case

[saberlynx]

Amended FilterTerm to change all filter keywords to lowercase.
Checklist:

• Tests
• CHANGELOG Entry

[swaterkamp]

Could you add a changelog entry for that and edit the PR's checklist accordingly?

[codecov]

Codecov Report

Merging #1444 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1444   +/-   ##
=======================================
  Coverage   36.71%   36.71%           
=======================================
  Files         989      989           
  Lines       22185    22185           
  Branches     6087     6110   +23     
=======================================
  Hits         8145     8145           
  Misses      12715    12715           
  Partials     1325     1325

Impacted Files	Coverage Δ
gsa/src/gmp/models/filter/filterterm.js	100% <100%> (ø)	⬆️

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[bjoernricks]

Could you do me a favor and add two additional tests?

• for Filter.fromString (see https://github.com/greenbone/gsa/blob/master/gsa/src/gmp/models/__tests__/filter.js#L24)
• for parsing filter from keywords (see https://github.com/greenbone/gsa/blob/master/gsa/src/gmp/models/__tests__/filter.js#L109)