deps: update node docker tag to v21

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
node	final	major	20-slim -> 21-slim

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Release Notes

nodejs/node (node)

v21

Moved to doc/changelogs/CHANGELOG_V4.md#4.2.6.

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Configuration

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[tophat-opensource-bot]

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This Pull Request introduces the following changes:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.84%. Comparing base (a2c3491) to head (7e8e0db). Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #898   +/-   ##
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  Coverage   91.84%   91.84%           
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  Files          55       55           
  Lines        1814     1814           
  Branches      496      496           
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  Hits         1666     1666           
  Misses        148      148