CI: Only mark PRs as stale

[ajgajg1134]

What does this PR do?

Need to manually set issue-stale to -1 so that we don't mark issues as stale (We only want to mark PRs as stale)

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-11 14:24:26

Comparing candidate commit 4480c70 in PR branch andrew.glaude/fixStale with baseline commit ef3fbf1 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[katiehockman]

As an alternative, I believe you can use the on field at the top to limit this to only PRs.
e.g. https://github.com/DataDog/dd-trace-go/blob/main/.github/workflows/ecosystems-label.yml#L2