Remove unused indexes from members table

What does this MR do and why?

• Remove unused indexes from members table
• Remove two unused temporary indexes from members table, index_project_members_on_id_temp and tmp_index_for_null_member_namespace_id
• tmp_index_for_null_member_namespace_id has no no usage in the last 6 months and index_project_members_on_id_temp has very little usage in the last 6 months.
• They were both created for background migrations 3 years ago. The background migrations also have been cleaned up - !99212 (merged) & !95881 (merged).

Database

main: == [advisory_lock_connection] object_id: 151520, pg_backend_pid: 32990
main: == 20250827050123 RemoveIndexProjectMembersOnIdTemp: migrating ================
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0378s
main: -- indexes(:members)
main:    -> 0.0087s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- remove_index(:members, {:algorithm=>:concurrently, :name=>"index_project_members_on_id_temp"})
main:    -> 0.0028s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20250827050123 RemoveIndexProjectMembersOnIdTemp: migrated (0.0664s) =======

main: == [advisory_lock_connection] object_id: 151520, pg_backend_pid: 32990

main: == [advisory_lock_connection] object_id: 151520, pg_backend_pid: 32993
main: == 20250827050202 RemoveTmpIndexForNullMemberNamespaceId: migrating ===========
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0004s
main: -- indexes(:members)
main:    -> 0.0061s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:members, {:algorithm=>:concurrently, :name=>"tmp_index_for_null_member_namespace_id"})
main:    -> 0.0029s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20250827050202 RemoveTmpIndexForNullMemberNamespaceId: migrated (0.0196s) ==

main: == [advisory_lock_connection] object_id: 151520, pg_backend_pid: 32993

How to set up and validate locally

MR acceptance checklist

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