feat: add support for Guzzle 7

[bednar]

Closes #47

Proposed Changes

• Add support for Guzzle 7
• Add CI build for Guzzle 7

Checklist

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

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 0.02%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master      #49      +/-   ##
============================================
+ Coverage     82.70%   82.73%   +0.02%     
- Complexity      349      350       +1     
============================================
  Files            18       18              
  Lines           798      805       +7     
============================================
+ Hits            660      666       +6     
- Misses          138      139       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/InfluxDB2/DefaultApi.php	83.33% <85.71%> (+0.31%)	17.00 <0.00> (+1.00)

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