feat: Add initial-version input to get-version workflow

[shiipou]

About this PR

This PR add initial-version input to allow to change the initial version calculated by convco.
That version will be 1.0.0 by default.

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• I didn't over-scope my PR
• My PR title matches the commit convention
• I did not include breaking changes
• I made my own code-review before requesting one

I included unit tests that cover my changes

• 👍 yes
• 🙅 no, because they aren't needed
• 🙋 no, because I need help

I added/updated the documentation about my changes

• 📜 README.md
• 📕 docs/*.md
• 📓 docs.lenra.io
• 🙅 no documentation needed

[Copilot]

Pull Request Overview

Adds an initial-version input parameter to the get-version workflow to allow customization of the initial version calculated by convco, defaulting to '1.0.0'.

• Added a new initial-version input parameter to the workflow with a default value of '1.0.0'
• Configured the convco CLI to use this custom initial version via the CONVCO_INITIAL_BUMP_VERSION environment variable

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