Fix partition_id for ci_pipeline_variables

Ref: #387301 (closed)

What does this MR do and why?

This MR fixes invalid ci_pipeline_variables records in production and sets the partition_id to 100.

Database

We have around ~315K records to fix.

Following a similar strategy used in !111287 (merged), I choose a default batch size of 2_000 and a sub batch size of 200.

select count(*) from ci_pipeline_variables where partition_id = 101;

📝 This seems to be the query executed by our background migration and its associated query plan.

UPDATE ci_pipeline_variables
SET partition_id = 100
WHERE ci_pipeline_variables.id BETWEEN 100000 AND 100200
AND ci_pipeline_variables.partition_id = 101

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