infra: set github actions max-parallel to 15 (#565)

this is the guidance from apache infra, see
https://s.apache.org/max-parallel
* All workflows MUST have a job concurrency level less than or equal to
20. This means a workflow cannot have more than 20 jobs running at the
same time across all matrices.
* All workflows SHOULD have a job concurrency level less than or equal
to 15. Just because 20 is the max, doesn't mean you should strive for
20.

note that `max-parallel` limits the number of simultaneous runs ([see
docs](https://docs.github.com/en/actions/how-tos/write-workflows/choose-what-workflows-do/run-job-variations#defining-the-maximum-number-of-concurrent-jobs)),
other jobs will just wait

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: kevinjqliu <9057843+kevinjqliu@users.noreply.github.com>

Author: kevinjqliu

.github/workflows/rc.yml

@@ -71,6 +71,7 @@ jobs:
       - archive
     runs-on: ${{ matrix.os }}
     strategy:
+      max-parallel: 15
       fail-fast: false
       matrix:
         os:

.github/workflows/test.yml

@@ -109,6 +109,7 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
     timeout-minutes: 30
     strategy:
+      max-parallel: 15
       fail-fast: false
       matrix:
         include: