Cleanup access to "otel.javaagent.*" / rename `ConfigPropertiesUtil`

[zeitlinger]

With #15787 there are only a few places left that access "otel.javaagent.*" directly and that access ConfigPropertiesUtil

• copy ConfigPropertiesUtil to a DeprecatedConfigPropertiesUtil and change all usages unless noted otherwise below
  • DeprecatedConfigPropertiesUtil will be removed in 3.0 (remove system property access from library instrumentation #15810)
• otel.javaagent.testing.*
  • should keep access via system properties, because we also want to test with declarative config without having to add to the yaml file
  • most access via System.getProperty directly
  • some use ConfigPropertiesUtil - change for consistency
• otel.javaagent.debug
  • in ContextPropagationDebug (instrumentation-api) - can't access EarlyInitAgentConfig
  • in SupportabilityMetrics (instrumentation-api) - can't access EarlyInitAgentConfig
• otel.javaagent.experimental.thread-propagation-debugger.enabled
  • should be renamed to be have otel.javaagent.testing. prefix
• otel.instrumentation.internal-reflection.enabled
  • use otel.javaagent.internal-reflection.enabled (access via EarlyInitAgentConfig) as this is a property that must be available early in startup
• otel.javaagent.experimental.initializer.jar
  • keep using ConfigPropertiesUtil
• otel.semconv-stability.opt-in
  • keep using ConfigPropertiesUtil
  • need to decide is this should be configurable in yaml file: support OTEL_SEMCONV_STABILITY_OPT_IN ? opentelemetry-configuration#492