Change milestone in migration docs to strings

What does this MR do and why?

Related to #442440 (closed)

Ran into an issue when working on a Housekeeper keep to read in YAML and record when the migration was marked as obsolete.

The YAML files need to use strings around the milestones otherwise this YAML

milestone: 15.10

becomes this when the file is read in using YAML.load_file

milestone: 15.1

This MR does two things:

1. update all existing docs files with the new format
2. updates the elastic-migration script to stringify the milestone

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

run the script and verify the milestone is quoted with strings

➜ scripts/elastic-migration

>> Name of the migration in CamelCase:
?> TerriTest

>> Description of what the migration does:
?> Test migration

>> The group introducing a feature flag, like: `global search`:
?> global search

>> URL of the MR introducing the migration (enter to skip):
?>
created ee/elastic/migrate/20240423113557_terri_test.rb
created ee/spec/elastic/migrate/20240423113557_terri_test_spec.rb
created ee/elastic/docs/20240423113557_terri_test.yml

=> Please consult the documentation for Advanced Search Migrations: https://docs.gitlab.com/ee/development/search/advanced_search_migration_styleguide.html

output

---
name: TerriTest
version: '20240423113557'
description: Test migration
group: group::global search
milestone: '17.0'
introduced_by_url:
obsolete: false
marked_obsolete_by_url:
marked_obsolete_in_milestone: