Upgraded and fixed the PHP Example #7 for the custom php runtime

[majdarbash]

Issue #, if available:

Description of changes:

• Updated the dependency and switch package manager to DNS
  public.ecr.aws/lambda/provided is switched to Amazon Linux 2 image, which uses DNF as a package manager
• Upgraded the PHP custom runtime example to the current stable version of 8.3.1
• Modified the bootstrap's handler parsing, to support a fileName.handlerFunction format e.g. index.handler

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