infra: set github actions max-parallel to 15

[kevinjqliu]

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), other jobs will just wait

[wgtmac]

Thanks for adding this!