Change db:setup:geo to db:migrate:geo in Geo docs

What does this MR do and why?

Before !77269 (diffs), the db:setup:geo task was simply task setup: ['geo:db:schema:load', 'geo:db:seed']; but now the built-in Rails task for multiple DBs is ["db:create", :environment, "db:schema:load", :seed] which causes issues since it doesn't have permissions to actually create the database.

The database is created through a reconfigure, so this shouldn't be necessary, and just migrate would do; this MR updates the docs to that.

Screenshots or screen recordings

root@cat-geo-secondary-gitlab-rails-1:/var/opt/gitlab/geo-postgresql# gitlab-rake db:setup:geo
Checking and creating secrets
PG::InsufficientPrivilege: ERROR:  permission denied to create database
Couldn't create 'gitlabhq_geo_production' database. Please check your configuration.
rake aborted!
ActiveRecord::StatementInvalid: PG::InsufficientPrivilege: ERROR:  permission denied to create database
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:119:in `block in write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:112:in `block in read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:172:in `retry_with_backoff'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:110:in `read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:118:in `write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:88:in `method_missing'
/opt/gitlab/embedded/bin/bundle:23:in `load'
/opt/gitlab/embedded/bin/bundle:23:in `<main>'

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