chore: updated supported versions matrix for Mage-OS 2.1.0

[rhoerr]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mage-os/github-actions/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

No Mage-OS 2.1.0 version in the matrix

Fixes: N/A

What is the new behavior?

Adds Mage-OS 2.1.0 to the supported versions matrix.. To be merged upon release of 2.1, anticipated Tuesday 2026-01-20.

I also changed versions specifying composer 2.8 in tests to 2.9.3, per the recent security advisory.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Tests pass.

$ npm run build

> @mage-os/github-actions-supported-version@1.0.0 build
> npx esbuild --outfile=dist/index.js --platform=node --bundle --minify src/index.ts


  dist/index.js  488.3kb

⚡ Done in 12ms
$ npm run test

> @mage-os/github-actions-supported-version@1.0.0 test
> jest

...

Test Suites: 10 passed, 10 total
Tests:       69 passed, 69 total
Snapshots:   0 total
Time:        0.782 s, estimated 1 s
Ran all test suites.