Emit a warning when pg gem < 1.6.0 is used with PostgreSQL 18+

Split from rails#55282

This commit emits a warning when `pg` gem < 1.6.0 is used with PostgreSQL 18+
to prevent compatibility issues of PG::Connection#cancel.

PostgreSQL 18 (scheduled for release around September/October 2025) changes the format of the cancel request key
, which causes PG::Connection#cancel to fail with pg gem 1.5.9 and below.
This issue is already addressed in ged/ruby-pg#614 and the fix is included in pg gem v1.6.0 (currently released as v1.6.0.rc2).

Instead of requiring pg gem 1.6+ for all users when raising minimum supported PostgreSQL version to 10 discussed at rails#55282 ,
warnings are only shown to PostgreSQL 18+ + pg gem ≤ 1.5.9 users actually affected by the PG::Connection#cancel behavior.

Also skip the failing test if pg gem version < 1.6.0 and PostgreSQL version is 18+.

```
$ bundle info pg
  * pg (1.5.9)
    Summary: Pg is the Ruby interface to the PostgreSQL RDBMS
    Homepage: https://github.com/ged/ruby-pg
    Documentation: http://deveiate.org/code/pg
    Source Code: https://github.com/ged/ruby-pg
    Changelog: https://github.com/ged/ruby-pg/blob/master/History.md
    Path: /home/yahonda/.local/share/mise/installs/ruby/3.4.4/lib/ruby/gems/3.4.0/gems/pg-1.5.9
    Reverse Dependencies:
        queue_classic (4.0.0) depends on pg (>= 1.1, < 2.0)
$
$ ARCONN=postgresql bin/test test/cases/adapters/postgresql/transaction_test.rb -n test_raises_Interrupt_when_canceling_statement_via_interrupt
Using postgresql
Run options: -n test_raises_Interrupt_when_canceling_statement_via_interrupt --seed 17678

F

Failure:
ActiveRecord::PostgresqlTransactionTest#test_raises_Interrupt_when_canceling_statement_via_interrupt [test/cases/adapters/postgresql/transaction_test.rb:199]:
Expected 10.014372003 to be < 5.

bin/test test/cases/adapters/postgresql/transaction_test.rb:183

Finished in 10.067954s, 0.0993 runs/s, 0.1987 assertions/s.
1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
```

Author: yahonda

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Emit a warning for pg gem < 1.6.0 when using PostgreSQL 18+
+
+    *Yasuo Honda*
+
 *   Fix `#merge` with `#or` or `#and` and a mixture of attributes and SQL strings resulting in an incorrect query.
 
     ```ruby

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -666,6 +666,9 @@ def check_version # :nodoc:
         if database_version < 9_03_00 # < 9.3
           raise "Your version of PostgreSQL (#{database_version}) is too old. Active Record supports PostgreSQL >= 9.3."
         end
+        if database_version >= 18_00_00 && Gem::Version.new(PG::VERSION) < Gem::Version.new("1.6.0")
+          warn "pg gem version #{PG::VERSION} is known to be incompatible with PostgreSQL 18+. Please upgrade to pg 1.6.0 or later."
+        end
       end
 
       class << self

activerecord/test/cases/adapters/postgresql/transaction_test.rb

@@ -181,6 +181,9 @@ class Sample < ActiveRecord::Base
     end
 
     test "raises Interrupt when canceling statement via interrupt" do
+      if ActiveRecord::Base.lease_connection.database_version >= 18_00_00 && Gem::Version.new(PG::VERSION) < Gem::Version.new("1.6.0")
+        skip "pg gem version #{PG::VERSION} is known to be incompatible with PostgreSQL 18+. "
+      end
       start_time = Time.now
       thread = Thread.new do
         Sample.transaction do