Fix imported project labels having a group_id

What does this MR do and why?

When a project is imported, we turn associated group labels into project labels. We need to clear the group_id because project labels must not have a group_id value.

Migration Output

main: == 20230217040544 AddTempIndexForInvalidProjectLabels: migrating ==============
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.2475s
main: -- index_exists?(:labels, :id, {:where=>"type = 'ProjectLabel' AND group_id IS NOT NULL", :name=>"tmp_idx_invalid_project_labels", :algorithm=>:concurrently})
main:    -> 0.0185s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- add_index(:labels, :id, {:where=>"type = 'ProjectLabel' AND group_id IS NOT NULL", :name=>"tmp_idx_invalid_project_labels", :algorithm=>:concurrently})
main:    -> 0.0061s
main: -- execute("RESET statement_timeout")
main:    -> 0.0008s
main: == 20230217040544 AddTempIndexForInvalidProjectLabels: migrated (0.3088s) =====

main: == 20230217054346 FixProjectLabelsWithGroupId: migrating ======================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- exec_query("SELECT COUNT(*) AS count FROM \"labels\" WHERE \"labels\".\"type\" = 'ProjectLabel' AND \"labels\".\"group_id\" IS NOT NULL")
main:    -> 0.0024s
main: == 20230217054346 FixProjectLabelsWithGroupId: migrated (0.0098s) =============

main: == 20230217070812 RemoveTempIndexForInvalidProjectLabels: migrating ===========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0007s
main: -- indexes(:labels)
main:    -> 0.0479s
main: -- remove_index(:labels, {:algorithm=>:concurrently, :name=>"tmp_idx_invalid_project_labels"})
main:    -> 0.0027s
main: == 20230217070812 RemoveTempIndexForInvalidProjectLabels: migrated (0.0634s) ==

MR acceptance checklist

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Related to #291034 (closed)