Cast ENV['RSPEC_PROFILING_POSTGRES_URL'] to symbol in establish_connection call …

What does this MR do?

Casts ENV['RSPEC_PROFILING_POSTGRES_URL'] to symbol in establish_connection call.

DEPRECATION WARNING: Passing a string to ActiveRecord::Base.establish_connection for a configuration lookup is deprecated, please pass a symbol (:"") instead. (called from establish_connection at /builds/gitlab-org/gitlab-ce/config/initializers/rspec_profiling.rb:4)

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Why was this MR needed?

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

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