fix: use pandas.concat() instead of deprecated DataFrame.append()

[bednar]

Closes #406

Proposed Changes

Use pandas.concat() instead of deprecated DataFrame.append():

https://pandas.pydata.org/docs/dev/whatsnew/v1.4.0.html#deprecated-dataframe-append-and-series-append

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• pytest tests completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #407 (3f82572) into master (52a9d02) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #407   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files          28       28           
  Lines        2506     2506           
=======================================
  Hits         2337     2337           
  Misses        169      169           

Impacted Files	Coverage Δ
influxdb_client/client/flux_csv_parser.py	98.00% <100.00%> (ø)

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