.github/workflows: increase operations-per-run to 100 in stale bot

.github/workflows: increase operations-per-run to 100 in stale bot

The stale bot's `operations-per-run`
(https://github.com/actions/stale#operations-per-run) corresponds to the max
number of API calls it is allowed to make per hour. Currently,
`operations-per-run` is set to 30, which means that the stale bot
can make up to 30 API calls per hour.

With this limit in place, the stale bot is only able to process 400 PRs at a time.
Since there are 900+ PRs in the Ceph repository, we should increase the number of
operations to cover them all. This needs to be done with care though, since GitHub
has a rate limit
(https://docs.github.com/en/rest/overview/resources-in-the-rest-api#rate-limiting)
depending on business plan.

According to GitHub's documentation on GitHub action requests
(https://docs.github.com/en/rest/overview/resources-in-the-rest-api#requests-from-github-actions),
the rate limit is 1,000 requests per hour per repository when using `GITHUB_TOKEN` (which we are).
For enterprise accounts, GitHub Enterprise Cloud's rate limit applies, and the limit is 15,000
requests per hour per repository.

Based on this information, we should be fine to increase the max `operations-per-run`
to 100. This would cover a little over 1000 PRs, which should be enough
to process the 900-some-odd PRs in the Ceph repository.

Signed-off-by: Laura Flores <lflores@redhat.com>