Nullify ci_build_id in pages_deployments table when build is destroyed

What does this MR do?

#336442 (closed)

• I can't use foreign key, as we're planning to move ci_builds to another database
• existing orphaned pages deployments will be fixed in another MR(to speed up reviews) Currently there are 0 orphaned records in the database(run explain SELECT "pages_deployments".* FROM "pages_deployments" LEFT JOIN ci_builds ON pages_deployments.ci_build_id = ci_builds.id WHERE pages_deployments.ci_build_id IS NOT NULL AND ci_builds.id IS NULL in #database-lab, and see Gather (cost=1001.00..31476.09 rows=1 width=133) (actual time=57.163..70.246 rows=0 on the top node
• We nullify, not delete records because this field on pages deployment is used only by !67303 (merged) and it handles nulls properly

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #336442 (closed)