more fixes to stac-model's readme

[fmigneault]

Description

• Releases badge were including the MLM specification instead of only stac-model-v... versions.
• CI workflow was using an incorrect file
• Using literal UTF-8 representation of the emojis, rather than GitHub :<name>: since they are not interpreted by PyPI

Related Issue

Follow up on #22

Type of Change

• 📚 Examples, docs, tutorials or dependencies update;
• 🔧 Bug fix (non-breaking change which fixes an issue);
• 🥂 Improvement (non-breaking change which improves an existing feature);
• 🚀 New feature (non-breaking change which adds functionality);
• 💥 Breaking change (fix or feature that would cause existing functionality to change);
• 🔐 Security fix.

Checklist

• I've read the CONTRIBUTING.md guide;
• I've updated the code style using make check;
• I've written tests for all new methods and classes that I created;
• I've written the docstring in Google format for all the methods and classes that I used.

[rbavery]

woohoo emojis! looks good.