support PG 15 and bump upgrade script (#100)

* support PG 15 and bump upgrade script
* increase health interval for Gh workflows
* increase health timeout for Gh workflows
* remove 10 and use postgis 3.3
* use correct file name in test script
* bump to 0.7.4
* test upgrade with 0.7.3 to 0.7.4
* set release date for 0.7.4 in doc

include minor change in reinit, too

update install docs

update version everywhere in META.json

Merge pull request #60 from pgMemento/update-docs-readme

Update docs readme

updated docs once more

updated docs

added link to pgMemento paper

some more formatting to README

tagged version 0.3#