Draft: Remove trigram indexes on issues.title/description for gitlab.com

What does this MR do and why?

Drop the trigram indexes on issues.description and issues.title, but only when run on gitlab.com.

These indexes contribute to very long autovaccuum times.

Relates to #331829

Migration output

(Run locally with Gitlab.com? changed to return true)

== 20211008160040 RemoveIssuesTrigramIndex: migrating =========================
-- transaction_open?()
   -> 0.0000s
-- indexes(:issues)
   -> 0.0102s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:issues, {:algorithm=>:concurrently, :name=>"index_issues_on_description_trigram"})
   -> 0.0037s
-- execute("RESET statement_timeout")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issues)
   -> 0.0075s
-- remove_index(:issues, {:algorithm=>:concurrently, :name=>"index_issues_on_title_trigram"})
   -> 0.0020s
== 20211008160040 RemoveIssuesTrigramIndex: migrated (0.0274s) ================

== 20211008160040 RemoveIssuesTrigramIndex: reverting =========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, :title, {:name=>"index_issues_on_title_trigram", :using=>:gin, :opclass=>{:content=>:gin_trgm_ops}, :algorithm=>:concurrently})
   -> 0.0087s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:issues, :title, {:name=>"index_issues_on_title_trigram", :using=>:gin, :opclass=>{:content=>:gin_trgm_ops}, :algorithm=>:concurrently})
   -> 0.0047s
-- execute("RESET statement_timeout")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issues, :description, {:name=>"index_issues_on_description_trigram", :using=>:gin, :opclass=>{:content=>:gin_trgm_ops}, :algorithm=>:concurrently})
   -> 0.0078s
-- add_index(:issues, :description, {:name=>"index_issues_on_description_trigram", :using=>:gin, :opclass=>{:content=>:gin_trgm_ops}, :algorithm=>:concurrently})
   -> 0.0042s
== 20211008160040 RemoveIssuesTrigramIndex: reverted (0.0293s) ================

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