Add net-smtp for Ruby 3.1 or higher (#65)

I got the following error in Ruby 3.1.

```
ruby -v
ruby 3.1.4p223 (2023-03-30 revision 957bb7cb81) [x86_64-linux]
```

```
/app/lib/groonga-query-log/command/run-regression-test.rb:24:in `require': cannot load such file -- net/smtp (LoadError)
	from /app/lib/groonga-query-log/command/run-regression-test.rb:24:in `<top (required)>'
	from bin/groonga-query-log-run-regression-test:20:in `require'
	from bin/groonga-query-log-run-regression-test:20:in `<main>'
```

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

.github/workflows/test.yml

@@ -13,6 +13,9 @@ jobs:
  - "2.6"
  - "2.7"
  - "3.0"
+ - "3.1"
+ - "3.2"
+ - "3.3"
  steps:
  - uses: actions/checkout@v4
  - name: Set up Groonga APT repository

groonga-query-log.gemspec

@@ -54,6 +54,7 @@ Gem::Specification.new do |spec|
 
  spec.add_runtime_dependency("charty")
  spec.add_runtime_dependency("diff-lcs")
+ spec.add_runtime_dependency("net-smtp")
  spec.add_runtime_dependency("groonga-client", ">= 0.6.2")
  spec.add_runtime_dependency("groonga-log", ">= 0.1.2")