[Improvement] Optimize ServerLoadProtection.

[Zzih96]

Purpose of the pull request

• Fix the issue where configurations couldn't load properly due to case sensitivity
• add configuration validation
• and modify error log messages.

Brief change log

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This pull request is code cleanup without any test coverage.

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[ruanwenjun]

Are you sure there will exist case sensitivity problem? add configuration LGTM, some change should already exist in #15803

[ruanwenjun]

Please resolve the conflicts.

[Zzih96]

LGTM

In the Kubernetes environment, I couldn't inject MASTER_SERVER_LOAD_PROTECTION_MAX_JVM_MEMORY_USAGE_PERCENTAGE_THRESHOLDS intomaxJVMMemoryUsagePercentageThresholds as configured. However, modifying it tomaxJvmMemoryUsagePercentageThresholds does allow successful injection. I suspect this might be related to the getters and setters generated by Lombok.

[Zzih96]

Please resolve the conflicts.

I'll give it a try.