Rails/StripHeredoc bad autocorrect when squiggly already present

Consider
```ruby
s = <<~EOF.strip_heredoc
  foo
EOF
```

Rails/StripHeredoc autocorrects this to:
```ruby
s = <<~~EOF
  foo
EOF
```

Expected:
```ruby
s = <<~EOF
  foo
EOF
```

Rubocop:
```sh
1.30.1 (using Parser 3.1.2.0, rubocop-ast 1.18.0, running on ruby 2.6.9 x86_64-darwin21)
  - rubocop-minitest 0.20.1
  - rubocop-performance 1.14.2
  - rubocop-rails 2.15.1
```