[README.md] Fix minimal PHP version badge

[iamluc]

Description

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.09%. Comparing base (9d93b40) to head (738cb3a).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2920      +/-   ##
============================================
+ Coverage     78.24%   82.09%   +3.85%     
  Complexity     2526     2526              
============================================
  Files           135      108      -27     
  Lines         14400    10360    -4040     
  Branches        989        0     -989     
============================================
- Hits          11267     8505    -2762     
+ Misses         2591     1855     -736     
+ Partials        542        0     -542     

Flag	Coverage Δ
appsec-extension	?
tracer-php	82.09% <ø> (ø)

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see 27 files with indirect coverage changes

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