NOP GitlabSettings::Options#symbolize_keys!

What does this MR do and why?

NOP GitlabSettings::Options#symbolize_keys!

To avoid problems where code alters the GitlabSettings::Options internal hash keys, make it a No Operation method and raise an exception in test and development mode.

Context

The original intent was to replace the internal hash with HashWithIndifferentAccess, but it break a few flows on the initialization. So, for a first iteration, I'm decided to do the NOP on symbolize_keys!/deep_symbolize_keys!

Related to: #413017 (closed)

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