Ensure syntax highlighting limits are configurable

What does this MR do?

We have a length limit on the amount of text we syntax highlight when viewing a file. This is normally hard-coded to 512kb:

# lib/gitlab/highlight.rb
#
MAXIMUM_TEXT_HIGHLIGHT_SIZE = config.maximum_test_highlight_size || 1.megabyte

This MR will let you add a line in the config/gitlab.yml file to make it a custom value named: maximum_text_highlight_size_kilobytes

Related to #31065

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