fix: make Click flag_value defaults work across Click 8.1-8.3

[DavidMinarsch]

Proposed changes

Summary

Fix registry_flag() and remote_registry_flag() in click_utils.py to use default=None with functools.wraps normalizing wrappers, so the correct configured default is used regardless of Click version
Apply the same default=None + normalization pattern to update_package_versions.py and add comments to packages.py (which already used this pattern)
Add inline documentation explaining the workaround and when it can be removed (Click lower bound raised to >=8.3)
Context

Click <8.3 ignores default= on flag_value options and uses the last-applied decorator's flag_value instead. Click >=8.3 honours default= but treats booleans literally. Previous fixes (PR fetchai#839 using booleans, PR
fetchai#848 using string defaults) each only worked on one Click version. This fix works across the full supported range (>=8.1.0,<8.4.0).

Test plan

Verified registry_flag and remote_registry_flag tests pass on Click 8.1.0
Verified registry_flag and remote_registry_flag tests pass on Click 8.3.1
Verified aea init flag combinations work correctly on both versions
All linters pass (black, isort, flake8, mypy, pylint)
Hash checks pass

Types of changes

What types of changes does your code introduce to agents-aea?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
• I am making a pull request against the main branch (left side). Also you should start your branch off our main.
• Lint and unit tests pass locally with my changes and CI passes too
• I have added tests that prove my fix is effective or that my feature works
• I have checked that code coverage does not decrease.
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments