TiDB should be able to handle the case when `cpu.max` doesn't exist and the cgroup config doesn't have cpu controller

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Use tiup cluster to deploy a cluster in rockylinux 9.

2. What did you expect to see? (Required)

TiDB doesn't report any error.

3. What did you see instead (Required)

TiDB reports error that the cpu.max doesn't exist.

4. What is your TiDB version? (Required)

[YangKeao]

It's because systemd has a strange behavior: when none of the systemd services used CPUQuota, it'll not register cpu controller, and we'll not have cpu.max.

TiDB should allow the situation that the cpu.max doesn't exist, and don't report error or trigger the alert in alertmanager.

[YangKeao]

It's set to moderate because it'll cause noisy alerting. Ref #50468

[YangKeao]

A workaround is to set cpu_quota manually. It's supported in tiup:

tidb_servers:
- host: xxxxx
  resource_control:
    cpu_quota: "1600%"