Merge schema upgrade files

[julianbrost]

Before tagging v0.2.0, ensure that the individual schema upgrade files added during development are merged into a single 0.2.0.sql file:

• https://github.com/Icinga/icinga-notifications/tree/main/schema/pgsql/upgrades
• https://github.com/Icinga/icinga-notifications/tree/main/schema/mysql/upgrades