Fix link to ToolHive good-first-issues label

[danbarr]

• Label name changed since originally published
• Add no:assignee filter

Signed-off-by: Dan Barr 6922515+danbarr@users.noreply.github.com

[github-actions]

Missing Issue Link

This PR does not appear to reference a GitHub issue.
Please add a closing keyword (e.g. Closes #123) if this PR fully resolves the issue, or a non-closing reference (e.g. Part of #123) if it's one of several PRs implementing it.
Cross-repo references are also supported (e.g. Part of owner/repo#123) for issues tracked outside this repository.
Supported closing keywords: closes, fixes, resolves, close, fix, resolve (case insensitive)
Supported non-closing references: relates to, related to, part of, refs, ref, references, reference, contributes to, follow-up to (case insensitive)
Note: closing keywords only auto-close issues in this repository — cross-repo references never auto-close, regardless of keyword.