Update string datapoint constraints to be in line with API changes

[ozangoktan]

This PR updates string datapoint constraints to be in line with the recent changes to the API, where values are limited by UTF-8 bytes rather than string length. Also cuts a minor release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.59%. Comparing base (6e8a485) to head (eee8a0d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #486   +/-   ##
=======================================
  Coverage   80.59%   80.59%           
=======================================
  Files          43       43           
  Lines        4194     4194           
=======================================
  Hits         3380     3380           
  Misses        814      814           

Files with missing lines	Coverage Δ
cognite/extractorutils/__init__.py	100.00% <100.00%> (ø)
cognite/extractorutils/uploader/time_series.py	76.54% <100.00%> (ø)

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[ozangoktan]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the string datapoint validation to align with recent API changes, switching from a character length limit to a UTF-8 byte limit. The version is bumped to 7.9.0 to reflect this change. The changes are consistently applied across the implementation, tests, version files, and changelog. The implementation correctly uses len(value.encode('utf-8')) for validation, and the tests have been updated to use the new constant and logic. The pull request appears to be correct and complete.

[einarmo]

🦄