feat: add PHP 7.4 typehints, set PHP 7.4 as minimal supported PHP ver…

[Roman3349]

Proposed Changes

Added PHP 7.4 typehints, fixed most of issues reported by PHPStan, set PHP 7.4 as minimal supported PHP version.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• make test completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 85.09804% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.81%. Comparing base (d0029dc) to head (ec1e576).

Files with missing lines	Patch %	Lines
src/InfluxDB2/ClientOptionsUdp.php	56.75%	16 Missing ⚠️
src/InfluxDB2/UdpWriter.php	21.42%	11 Missing ⚠️
src/InfluxDB2/ClientOptions.php	91.37%	5 Missing ⚠️
src/InfluxDB2/DefaultApi.php	87.50%	3 Missing ⚠️
src/InfluxDB2/Client.php	87.50%	1 Missing ⚠️
src/InfluxDB2/FluxCsvParser.php	97.77%	1 Missing ⚠️
src/InfluxDB2/WriteApi.php	95.83%	1 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #178      +/-   ##
============================================
+ Coverage     74.86%   78.81%   +3.94%     
+ Complexity      461      450      -11     
============================================
  Files            25       27       +2     
  Lines          1110     1147      +37     
============================================
+ Hits            831      904      +73     
+ Misses          279      243      -36     

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