Update timestamp values for software license policies

What does this MR do?

Continuation of !22542 (merged). This MR fills NULL values for entities that are exists.

/chatops run explain SELECT count(*) FROM software_license_policies WHERE created_at IS NULL

Aggregate  (cost=137.72..137.73 rows=1 width=8) (actual time=4.492..4.492 rows=1 loops=1)
  Buffers: shared read=28
  I/O Timings: read=2.215
  ->  Seq Scan on software_license_policies  (cost=0.00..137.69 rows=13 width=0) (actual time=0.142..4.282 rows=2559 loops=1)
        Filter: (created_at IS NULL)
        Rows Removed by Filter: 28
        Buffers: shared read=28
        I/O Timings: read=2.215
Planning time: 0.873 ms
Execution time: 4.549 ms

It seems we have 2559 entities to be updated.

Screenshots

Some values set null before and after running migration they were updated.

Conformity

• Code review guidelines
• Database guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team