Enforce milestone validation before creating PlatformIO dependency PRs

[JanPetterMG]

This PR introduces a fail-fast milestone validation step in the dependency update workflows.

Before invoking peter-evans/create-pull-request, the workflow now verifies that:

• The configured milestone exists
• The milestone is open

If the milestone is missing or closed, the workflow fails immediately.

Rationale

Previously, PR creation could fail during the pull request creation step when:

• The milestone existed but was closed
• The milestone ID did not exist (common in forks)

This could lead to repeated failed runs and PR stacking behavior.

By validating the milestone explicitly and early, the workflow:

• Fails deterministically
• Avoids partial PR creation attempts
• Prevents unnecessary PR accumulation
• Makes milestone configuration errors immediately visible

This change improves workflow robustness, especially in fork scenarios where milestone numbers differ or are absent.