Rename legacy banzai filters

lib/banzai/filter/blockquote_fence_legacy_filter.rb

lib/banzai/filter/dollar_math_post_legacy_filter.rb

lib/banzai/filter/dollar_math_pre_legacy_filter.rb

lib/banzai/filter/table_of_contents_legacy_filter.rb

lib/banzai/filter/table_of_contents_tag_filter.rb

lib/banzai/pipeline/broadcast_message_pipeline.rb

lib/banzai/pipeline/gfm_pipeline.rb

lib/banzai/pipeline/note_pipeline.rb

lib/banzai/pipeline/plain_markdown_pipeline.rb

lib/banzai/pipeline/quick_action_pipeline.rb

spec/controllers/help_controller_spec.rb

spec/features/markdown/copy_as_gfm_spec.rb

spec/features/markdown/markdown_spec.rb

spec/fixtures/blockquote_fence_legacy_after.md

spec/fixtures/blockquote_fence_legacy_before.md

spec/fixtures/markdown.md.erb

.../lib/banzai/filter/blockquote_fence_legacy_filter_spec.rb

...lib/banzai/filter/table_of_contents_legacy_filter_spec.rb

spec/lib/banzai/pipeline/wiki_pipeline_spec.rb

spec/lib/gitlab/utils/mime_type_spec.rb