CI: Add PostgreSQL 15 and drop 9 and 10

ref: https://www.postgresql.org/support/versioning/

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Author: ashie

.github/workflows/linux.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby: [ '3.2', '3.1', '3.0', '2.7' ]
-        postgres: [ '14', '13', '12', '11', '10', '9' ]
+        postgres: [ '15', '14', '13', '12', '11' ]
         os:
           - ubuntu-latest
         experimental: [false]
@@ -34,14 +34,6 @@ jobs:
             postgres: '11'
             os: ubuntu-latest
             experimental: true
-          - ruby: head
-            postgres: '10'
-            os: ubuntu-latest
-            experimental: true
-          - ruby: head
-            postgres: '9'
-            os: ubuntu-latest
-            experimental: true
     services:
       postgres:
         image: postgres:${{ matrix.postgres }}