Clean up DAST site tokens when site deleted

What does this MR do and why?

this merge request ensures that dast_site_tokens and dast_site_validations are removed when dast_sites are deleted. this is necessary to avoid leaving orphaned dast_site_validations.

Related Issue(s)

Failed DAST site validations remain after deleting site profiles

Screenshots

How to set up and validate locally

1. start a rails console.

2. create some records.

dast_site = DastSite.create!(project: Project.last, url: 'https://example.com')
dast_site_token = DastSiteToken.create!(project: Project.last, url: dast_site.url, token: SecureRandom.uuid)

100.times { DastSiteValidation.create!(dast_site_token: dast_site_token, validation_strategy: :text_file, url_path: '/some_path') }

3. delete the the record and check the counts.

DastSiteValidation.count
DastSiteToken.count

dast_site.destroy!
# TRANSACTION (0.2ms)  BEGIN /*application:console,db_config_name:main,line:/lib/gitlab/database/load_balancing/connection_proxy.rb:119:in `block in write_using_load_balancer'*/
# DastSite Destroy (1.1ms)  DELETE FROM "dast_sites" WHERE "dast_sites"."id" = 32 /*application:console,db_config_name:main,line:/lib/gitlab/database/load_balancing/connection_proxy.rb:119:in `block in write_using_load_balancer'*/
# DastSiteToken Destroy (2.2ms)  DELETE FROM "dast_site_tokens" WHERE "dast_site_tokens"."project_id" = 21 AND "dast_site_tokens"."url" = 'https://example.com' /*application:console,db_config_name:main,line:/lib/gitlab/database/load_balancing/connection_proxy.rb:119:in `block in write_using_load_balancer'*/
# TRANSACTION (0.7ms)  COMMIT /*application:console,db_config_name:main,line:/lib/gitlab/database.rb:287:in `commit'*/

DastSiteValidation.count
DastSiteToken.count

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