databases/rubygem-activerecord-session_store: Update to 2.1.0

• Update RUN_DEPENDS: use Rails 6.1
• Change RUN_DEPENDS from rubygem-rack22 to rubygem-rack

Changes: https://github.com/rails/activerecord-session_store/releases
Security: CVE-2019-25025
(cherry picked from commit c46e478b82d1e4af2933655e7c0c1b913aba2de3)