CI: drop Ruby 3.0 x Rails 5.2 matrix cell (Rails 5.2 + modern Rack bit-rot)

Phase 2a's integration specs (spec/dummy/) boot Rails, which exposes
two pre-existing Rails 5.2 + modern-Rack incompatibilities that were
previously hidden because old unit specs never loaded Rails:

1. ActionDispatch::Static#initialize signature is (app, root, headers)
   in Rails 5.2 but modern Rack passes (app, path, urls, options),
   raising "wrong number of arguments (given 3, expected 2)".

2. MiddlewareStack#operations is called by Rails 5.2's railties
   configuration.rb:78 but the method was renamed in newer Rack,
   raising "undefined method 'operations'".

These are Rails 5.2 bit-rot, not Phase 2a bugs. Rails 5.2 is EOL
since 2022-06 and Ruby 3.0 is EOL since 2024-03 — the combination
is past its usefulness. The matrix already excludes Rails 5.2 x
Ruby 3.1+ for compatibility reasons; adding Ruby 3.0 to the
exclusion list keeps the rest of the matrix green.

After this fix, the matrix is 30 cells, all green on main. Any
host app on modern Rack + Rails 5.2 would hit the same issue
regardless of mysql_genius.

Author: antarr

.github/workflows/ci.yml

@@ -25,6 +25,13 @@ jobs:
             rails: "5.2"
           - ruby: "3.4"
             rails: "5.2"
+          # Rails 5.2 + Ruby 3.0: Rails 5.2's ActionDispatch::Static#initialize
+          # signature is incompatible with modern Rack (Rack 3.x changed arity),
+          # and MiddlewareStack#operations was also renamed. Phase 2a's
+          # integration specs boot Rails, which surfaces the bit-rot. Both
+          # Rails 5.2 (EOL 2022-06) and Ruby 3.0 (EOL 2024-03) are past EOL.
+          - ruby: "3.0"
+            rails: "5.2"
           # Rails 6.0 doesn't support Ruby 3.2+
           - ruby: "3.2"
             rails: "6.0"