Update mixin to latest changes from grafana/postgres_exporter (#1179)

* Update mixin to latest changes from grafana/postgres_exporter

Porting a bunch of PRs accumulated over time in grafana/postgres_exporter:

- https://github.com/grafana/postgres_exporter/pull/11 (@v-zhuravlev)
- https://github.com/grafana/postgres_exporter/pull/12 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/13 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/14 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/15 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/16 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/17 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/20 (@gaantunes)
- https://github.com/grafana/postgres_exporter/pull/21 (@mshahzeb)
- https://github.com/grafana/postgres_exporter/pull/22 (@mshahzeb)

Signed-off-by: Cristian Greco <cristian@regolo.cc>

* rename dashboard to old name

Signed-off-by: Cristian Greco <cristian@regolo.cc>

* remove custom selector

Signed-off-by: Cristian Greco <cristian@regolo.cc>

---------

Signed-off-by: Cristian Greco <cristian@regolo.cc>
Co-authored-by: Vitaly <v-zhuravlev@users.noreply.github.com>

postgres_mixin/.lint

@@ -0,0 +1,15 @@
+---
+exclusions:
+  panel-units-rule:
+    reason: Ignoring so far, need to address this in future
+  panel-title-description-rule:
+    reason: Ignoring so far, need to address this in future
+  panel-datasource-rule:
+    reason: "Loki datasource variable is being named as loki_datasource now while linter expects 'datasource'"
+  template-datasource-rule:
+    reason: "Based on new convention we are using variable names prometheus_datasource and loki_datasource where as linter expects 'datasource'"
+  alert-name-camelcase:
+    reason: QPS is a common acronym (Queries Per Second) and should be allowed
+    entries:
+      - alert: PostgreSQLQPS
+