RuboCop: Fix all offenses for Performance/StringIdentifierArgument

What does this MR do and why?

This MR fixes all offenses for 👮 Performance/StringIdentifierArgument.

Done via:

rubocop -A --only Performance/StringIdentifierArgument

Verified via:

rake rubocop:todo:generate[Performance/StringIdentifierArgument]

Thus, empty todo file removed.

Contributes to #239356

Micro benchmarks

TLDR; Using String is ~1.7x slower.

# frozen_string_literal: true

require "benchmark/ips"

o = Object.new

Benchmark.ips do |x|
  x.report "string" do
    o.instance_variable_defined?("@x")
  end

  x.report "symbol" do
    o.instance_variable_defined?(:@x)
  end

  x.compare!
end

Results

Warming up --------------------------------------
              string     1.400M i/100ms
              symbol     2.436M i/100ms
Calculating -------------------------------------
              string     13.907M (± 0.3%) i/s -     69.995M in   5.033067s
              symbol     24.228M (± 2.8%) i/s -    121.784M in   5.031130s

Comparison:
              symbol: 24227885.9 i/s
              string: 13907236.4 i/s - 1.74x  (± 0.00) slower

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.