Add os release exclusion mechanism into postgres-matrix.yml

Currently, we manage os/release-postgres match exclusion in pg_exclude.yml. We can merge this into postgres-matrix file as below

```yml
name: Postgres Version Matrix
project_name: citus

version_matrix:
- 8.0:
postgres_versions: [ 10, 11 ]
- 9.0:
postgres_versions: [ 11, 12 ]
- 9.5:
postgres_versions: [ 11, 12, 13 ]
- 10.1:
postgres_versions: [ 12, 13 ]
- 10.2:
postgres_versions: [ 12, 13, 14 ]
- 11.0:
postgres_versions: [ 13, 14 ]
- 11.1:
postgres_versions: [ 13, 14, 15 ]

exclude:
nightly:
ol/7: [ 15 ]
release:
all: [ 15 ]
```