Test on PostgreSQL 11 and POSTGIS 2.5.5

[billgeo]

User Story

In order to upgrade the infrastructure to POstgres 11, as a NZGB system owner, I want the automated testing passing on PostgresSQL 11 and POSTGIS 2.5.5.

Context: the infrastructure vendor is having trouble upgrading to version 12 and has suggested using version 11 in the meantime.

Acceptance Criteria

• [ ] Branch created with docker, makefile and CI files changed to these version
• [ ] All tests passing
• [ ] Communicate to @billgeo that it is passing
• [ ] Merge branch to master
• [ ] Run CI on multiple versions if possible, probably 11, 12 and 13 (see POSTGIS support matrix here https://trac.osgeo.org/postgis/wiki/UsersWikiPostgreSQLPostGIS)

Additional context

• [ ] Use one of these patterns in CI https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-including-additional-values-into-combinations