Add semver column to ci_runners table

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR is part of a sequence of MRs that will ultimately result in being able to efficiently count the number of out-of-date CI runners in a given scope (instance/namespace/project). Please refer to the table below for more details:

Sequence	MR	notes
1 ⬅	Add semver column to ci_runners table (!89023 - merged)	Adds a migration to add indexed machine-readable semver version text column (major.minor.patch) on top of the existing ci_runners.version column.
2	Store Runner semver value on version update (!89024 - merged)	Ensures that semver column is kept up-to-date based on version value
3	Backfill runner semver column (!89054 - merged)	Adds batched background migration to backfill semver column from version values.

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Part of #339523 (closed)