Fix MySQL expression index dumping with escaped quotes

[fatkodima]

Fixes #49334.
There was a "similar" issue previously #40822.

[Flixt]

I have a similar problem in #42429 with expressions of check constraints.
By the amount of occurrences of this problem I start wondering if there is a bigger problem somewhere on a lower level and if we are just fixing symptoms 🤔. Do you have any deeper understanding of why this happens at all?

[fatkodima]

For MySQL/PostgreSQL/SQLite in order to pass ' or " as a symbol in a string literal, we need to escape it (usually done by repeating the same character twice, like in 'O''Connor'), see

rails/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb

Lines 88 to 90 in 23f130c

	def quote_string(s)
	s.gsub("\\", '\&\&').gsub("'", "''") # ' (for ruby-mode)
	end

Then later, when we query these DBMS's for that value, it returns it quoted, as we passed it previously. If we will use these values as is, for example, in schema.rb and try to load the schema, rails will escape them again, which is incorrect. Thats why we need to remove these escapes, in all the places. I do not think there is a single place to do this.