standardize config on ruamel.yaml with a YAML 1.2 config

This is a key piece of the ongoing PUBLISH config.yml migration. There was uneven implementation of which YAML parser to use, and that could lead to bugs where one parser might read a value one way, and a different parser will read the value a different way. I wanted to be sure that YAML 1.2 would always work.

This makes all code that handles config files use the same ruamel.yaml parsers. This only touches other usages of YAML parsers when there is overlap. This does not port all of fdroidserver to ruamel.yaml and YAML 1.2. The metadata files should already be YAML 1.2 anyway.

Based on @jspricke's proposal in !1610 (comment 2380802213)