fix: declare the coder for the custom column

[danquinney]

Why

Apps that adopt config.load_defaults 7.1 get active_record.default_column_serializer = nil, and from Rails 7.1 serialize then insists on an explicit coder:

missing keyword: :coder If no default coder is configured, a coder must be provided to `serialize`.

Attach::Attachment declares serialize :custom, type: Hash, default: {} without one, and because Attach::Railtie loads the model during initialization the host app fails to boot. Found while moving Dial 9 Connect to load_defaults 7.1.

Change

serialize :custom, coder: YAML, type: Hash, default: {} — same on-disk format as today (YAML), just declared explicitly. Active Record < 7.1 ignores the keyword, exactly as it already ignores type: from 2.0.3, so the support floor is unchanged.

Verification

Second commit fixes the test setup so the CI matrix can actually run again (nothing else changed): MigrationContext.new only takes the schema migration class before 7.1, logger is required explicitly for Rails < 7.1 on concurrent-ruby ≥ 1.3.5, and the Rails 6 gemfile pins sqlite3 ~> 1.4 (the 6.1 adapter's requirement, which 2.x broke on Ruby ≥ 3.0).

Local runs of the matrix corners, all 40 examples, 0 failures:

Ruby	rails6.gemfile	rails7.gemfile
2.7.5	Active Record 6.1.7	Active Record 7.0.10
3.2.10	Active Record 6.1.7	Active Record 7.2.3

Also checked that booting the model with ActiveRecord::Base.default_column_serializer = nil (what load_defaults 7.1 does) loads and reports ActiveRecord::Coders::YAMLColumn as the custom coder with this change, and raises the error above without it.