Add default value to workspace.desired_config_generator_version

Issue: Drop the desired_config_generator_version colum... (#538166 - closed)

What does this MR do and why?

Add default value to workspace.desired_config_generator_version

This change is required because workspace.desired_config_generator_version does not have any default value on database level. This made Mark desired_config_generator_version ignore fo... (!205613 - merged) backwards incompatible.

Read the incident report for more details— gitlab-com/gl-infra/production#20594 (comment 2770476832)

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Before	After

How to set up and validate locally

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For DB reviewers

Query

UPDATE "workspaces"  SET "desired_config_generator_version" = 3  WHERE "workspaces"."desired_config_generator_version" IS NULL AND "workspaces"."id" >= 1 AND "workspaces"."id" < 50

Explain output

 ModifyTable on public.workspaces  (cost=0.29..3.30 rows=0 width=0) (actual time=0.022..0.023 rows=0 loops=1)
   Buffers: shared hit=5
   I/O Timings: read=0.000 write=0.000
   ->  Index Scan using workspaces_pkey on public.workspaces  (cost=0.29..3.30 rows=1 width=10) (actual time=0.021..0.021 rows=0 loops=1)
         Index Cond: ((workspaces.id >= 1) AND (workspaces.id < 50))
         Filter: (workspaces.desired_config_generator_version IS NULL)
         Rows Removed by Filter: 0
         Buffers: shared hit=5
         I/O Timings: read=0.000 write=0.000
Settings: jit = 'off', random_page_cost = '1.5', seq_page_cost = '4', work_mem = '100MB', effective_cache_size = '472585MB'

Statistics:

Time: 1.020 ms
  - planning: 0.915 ms
  - execution: 0.105 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 5 (~40.00 KiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

Query locks:

  # | RELKIND | RELNAMESPACE |                   RELNAME                    | BELONGS TO RELATION | LOCKTYPE |       MODE       | GRANTED | FASTPATH  
----+---------+--------------+----------------------------------------------+---------------------+----------+------------------+---------+-----------
  7 | r       | public       | workspaces                                   | workspaces          | relation | RowExclusiveLock | true    | true      
  2 | i       | public       | index_workspaces_on_project_id               | workspaces          | relation | RowExclusiveLock | true    | true      
  3 | i       | public       | index_workspaces_on_cluster_agent_id         | workspaces          | relation | RowExclusiveLock | true    | true      
  1 | i       | public       | index_workspaces_on_user_id                  | workspaces          | relation | RowExclusiveLock | true    | true      
  5 | i       | public       | index_workspaces_on_personal_access_token_id | workspaces          | relation | RowExclusiveLock | true    | true      
  6 | i       | public       | workspaces_pkey                              | workspaces          | relation | RowExclusiveLock | true    | true      
  4 | i       | public       | index_workspaces_on_name                     | workspaces          | relation | RowExclusiveLock | true    | true