Recreate async index `index_vuln_findings_on_uuid_including_vuln_id`

What does this MR do and why?

This migration was originally added for issue 390198. The migration ended up failing due to white-space being included in the SQL definition.

To prevent this going forward:

1. a before_validation callback was added to the async model class to strip whitespace
2. definition.to_s.strip was added to the migration helper as suggested by this comment

to correct the existing records in production

1. a migration was added that updates the existing records by calling strip on the definition column

---

Changelog: fixed

How to set up and validate locally

1. before running the migration, you can create an invalid record in the database with the following lambda:

   lambda {
     index_name = 'index_vuln_findings_on_uuid_including_vuln_id'
     index_sql = <<-SQL
       CREATE UNIQUE INDEX CONCURRENTLY #{index_name} ON vulnerability_occurrences (uuid) include (vulnerability_id);
     SQL
   
     async_index = Gitlab::Database::AsyncIndexes::PostgresAsyncIndex.find_or_create_by!(name: index_name) do |rec|
       rec.table_name = :vulnerability_occurrences
       rec.definition = index_sql
     end
   
     async_index.update_column(:definition, index_sql)
   }.call

2. check that an async index was created and that the definition field starts with whitespace

   [3] pry(main)> Gitlab::Database::AsyncIndexes::PostgresAsyncIndex.all
   
   => [#<Gitlab::Database::AsyncIndexes::PostgresAsyncIndex:0x00007fdf6021b2d0
     id: 10,
     created_at: Mon, 10 Jul 2023 23:38:47.973580000 UTC +00:00,
     updated_at: Mon, 10 Jul 2023 23:38:47.973580000 UTC +00:00,
     name: "index_vuln_findings_on_uuid_including_vuln_id",
     definition: "    CREATE UNIQUE INDEX CONCURRENTLY index_vuln_findings_on_uuid_including_vuln_id ON vulnerability_occurrences (uuid) include (vulnerability_id);\n",
     table_name: "vulnerability_occurrences",
     attempts: 0,
     last_error: nil>]

3. run the migration: bundle exec rails db:migrate
4. check the db again, the whitespace should have been removed:

   [1] pry(main)> Gitlab::Database::AsyncIndexes::PostgresAsyncIndex.all
   
   => [#<Gitlab::Database::AsyncIndexes::PostgresAsyncIndex:0x00007ff58c287718
     id: 10,
     created_at: Mon, 10 Jul 2023 23:38:47.973580000 UTC +00:00,
     updated_at: Mon, 10 Jul 2023 23:53:44.272120000 UTC +00:00,
     name: "index_vuln_findings_on_uuid_including_vuln_id",
     definition: "CREATE UNIQUE INDEX CONCURRENTLY index_vuln_findings_on_uuid_including_vuln_id ON vulnerability_occurrences (uuid) include (vulnerability_id);",
     table_name: "vulnerability_occurrences",
     attempts: 0,
     last_error: nil>]
   [2] pry(main)> 
   

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #413010 (closed)