Swap to unique endpoint identifier for prometheus alert integrations

What does this MR do and why?

Related issue: #338838 (closed)

In advance of migrating Integrations::Prometheus records to AlertManagement::HttpIntegrations, this sets up the HttpIntegration class to expect an endpoint identifier of 'legacy-prometheus' instead of the existing 'legacy'. This allows future migrated records to adhere to the unique index on the table.

How to set up and validate locally

Testing/validation is through rails console only, as no legacy prometheus integrations have been migrated/created.

You can check output URLs like this:

[1] pry(main)> integration = AlertManagement::HttpIntegration.new(project: Project.last, name: 'Name', endpoint_identifier: 'legacy')
  Project Load (2.3ms)  SELECT "projects".* FROM "projects" ORDER BY "projects"."id" DESC LIMIT 1 /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:(pry):1:in `__pry__'*/
=> #<AlertManagement::HttpIntegration:0x000000011b605178
 id: nil,
 created_at: nil,
 updated_at: nil,
 project_id: 9,
 active: false,
 encrypted_token: nil,
 encrypted_token_iv: nil,
 endpoint_identifier: "legacy",
 name: "Name",
 payload_example: {},
 payload_attribute_mapping: {},
 type_identifier: "http",
 token: nil>
[2] pry(main)> integration.url
  Namespace Load (2.4ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = 47 LIMIT 1 /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:/config/routes.rb:311:in `block (4 levels) in <main>'*/
  Feature::FlipperGate Load (0.5ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'cached_route_lookups' /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:/lib/feature.rb:237:in `block in current_feature_value'*/
  Route Load (4.2ms)  SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 47 AND "routes"."source_type" = 'Namespace' LIMIT 1 /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:/app/models/concerns/routable.rb:141:in `block in full_attribute'*/
=> "http://gdk.test:3000/namespace1/project-1/alerts/notify.json"
[3] pry(main)> integration = AlertManagement::HttpIntegration.new(project: Project.last, name: 'Name', endpoint_identifier: 'legacy-prometheus')
  Project Load (2.0ms)  SELECT "projects".* FROM "projects" ORDER BY "projects"."id" DESC LIMIT 1 /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:(pry):3:in `__pry__'*/
=> #<AlertManagement::HttpIntegration:0x000000011cd86ae0
 id: nil,
 created_at: nil,
 updated_at: nil,
 project_id: 9,
 active: false,
 encrypted_token: nil,
 encrypted_token_iv: nil,
 endpoint_identifier: "legacy-prometheus",
 name: "Name",
 payload_example: {},
 payload_attribute_mapping: {},
 type_identifier: "http",
 token: nil>
[4] pry(main)> integration.url
  Namespace Load (1.2ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = 47 LIMIT 1 /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:/config/routes.rb:311:in `block (4 levels) in <main>'*/
  Route Load (0.5ms)  SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 47 AND "routes"."source_type" = 'Namespace' LIMIT 1 /*application:console,db_config_name:main,console_hostname:macbook-pro.lan,console_username:syasonik,line:/app/models/concerns/routable.rb:141:in `block in full_attribute'*/
=> "http://gdk.test:3000/namespace1/project-1/prometheus/alerts/notify.json"

MR acceptance checklist

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