Remove unused semver column on ci_runners

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR addresses #365250 (closed) by doing the following:

• removing the unused semver column and related index from the ci_runners table, introduced in !89023 (merged);

• removing the Gitlab::BackgroundMigration::BackfillCiRunnerSemver background migration, which had already been deleted in %15.2 by the DeleteBackfillCiRunnerSemverMigration post-migration (not sure if this is the correct way to do things, so would appreciate feedback);

• fixing the ignore_column milestone, which was set to the N+1 milestone instead of N+2, as specified in the documentation:

  

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How to set up and validate locally

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Database migration output

Migrate up

main: == 20220719165537 RemoveCiRunnersSemverColumn: migrating ======================
main: -- remove_column(:ci_runners, :semver)
main:    -> 0.0029s
main: == 20220719165537 RemoveCiRunnersSemverColumn: migrated (0.0036s) =============

ci: == 20220719165537 RemoveCiRunnersSemverColumn: migrating ======================
ci: -- remove_column(:ci_runners, :semver)
ci:    -> 0.0017s
ci: == 20220719165537 RemoveCiRunnersSemverColumn: migrated (0.0017s) =============

Migrate down

main: == 20220719165537 RemoveCiRunnersSemverColumn: reverting ======================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- add_column(:ci_runners, :semver, :text, {:null=>true})
main:    -> 0.0007s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- current_schema()
main:    -> 0.0001s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE ci_runners\nADD CONSTRAINT check_a4f24953fd\nCHECK ( char_length(semver) <= 16 )\nNOT VALID;\n")
main:    -> 0.0004s
main: -- current_schema()
main:    -> 0.0001s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- execute("ALTER TABLE ci_runners VALIDATE CONSTRAINT check_a4f24953fd;")
main:    -> 0.0002s
main: -- execute("RESET statement_timeout")
main:    -> 0.0001s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:ci_runners, "id, (semver::cidr)", {:name=>"index_ci_runners_on_id_and_semver_cidr", :algorithm=>:concurrently})
main:    -> 0.0043s
main: -- add_index(:ci_runners, "id, (semver::cidr)", {:name=>"index_ci_runners_on_id_and_semver_cidr", :algorithm=>:concurrently})
main:    -> 0.0019s
main: == 20220719165537 RemoveCiRunnersSemverColumn: reverted (0.0234s) =============

ci: == 20220719165537 RemoveCiRunnersSemverColumn: reverting ======================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- add_column(:ci_runners, :semver, :text, {:null=>true})
ci:    -> 0.0006s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- current_schema()
ci:    -> 0.0001s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- execute("ALTER TABLE ci_runners\nADD CONSTRAINT check_a4f24953fd\nCHECK ( char_length(semver) <= 16 )\nNOT VALID;\n")
ci:    -> 0.0004s
ci: -- current_schema()
ci:    -> 0.0001s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0001s
ci: -- execute("ALTER TABLE ci_runners VALIDATE CONSTRAINT check_a4f24953fd;")
ci:    -> 0.0004s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0001s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- index_exists?(:ci_runners, "id, (semver::cidr)", {:name=>"index_ci_runners_on_id_and_semver_cidr", :algorithm=>:concurrently})
ci:    -> 0.0041s
ci: -- add_index(:ci_runners, "id, (semver::cidr)", {:name=>"index_ci_runners_on_id_and_semver_cidr", :algorithm=>:concurrently})
ci:    -> 0.0013s
ci: == 20220719165537 RemoveCiRunnersSemverColumn: reverted (0.0206s) =============

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