Add SEVERITY_AT_LEAST to get_alert_condition_from_string

[saberlynx]

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #268 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #268   +/-   ##
=======================================
  Coverage   96.88%   96.88%           
=======================================
  Files          18       18           
  Lines        3753     3755    +2     
=======================================
+ Hits         3636     3638    +2     
  Misses        117      117           

Impacted Files	Coverage Δ
gvm/protocols/gmpv7/types.py	100.00% <100.00%> (ø)

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