fix: correct misleading error message for undefined 'apikey'

[armin-x86]

This PR fixes a misleading error message in the codebase. Previously, the error message referenced "ApiKey", which could mislead users into thinking they need to use uppercase letters in their configuration. The error message now correctly references "apikey", aligning with the Fluent Bit documentation and actual configuration syntax.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change:
  N/A (This change affects only the error message and does not introduce functional changes.)
• Debug log output from testing the change:
  Log output after the change:
  [error] no apikey configuration key defined
• Attached Valgrind output that shows no leaks or memory corruption was found:
  N/A (This change only updates a string in the error message.)

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• Run local packaging test showing all targets (including any new ones) build.
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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[cosmo0920]

This PR fixes a misleading error message in the codebase. Previously, the error message referenced "ApiKey", which could mislead users into thinking they need to use uppercase letters in their configuration. The error message now correctly references "apikey", aligning with the Fluent Bit documentation and actual configuration syntax.

Our configuration system is case-insensitive. So, it's not correct understandings. Using CamelCase in the error message is just for emphasizing the value of configuration key.