Fix up timestamp quoting issue.

[dereuromark]

Resolves cakephp/phinx#1891

When setting 'CURRENT_TIMESTAMP' as a default value on non-datetime columns (like VARCHAR/TEXT), it was not being quoted properly, causing SQL errors.
The logic was inverted - it would only quote strings that DON'T start with CURRENT_TIMESTAMP, when it should only skip quoting for datetime-related columns.

with the buggy code, these scenarios would FAIL:

1. ❌ 'CURRENT_TIMESTAMP' on STRING column → would output DEFAULT CURRENT_TIMESTAMP (unquoted - invalid SQL)
2. ❌ 'CURRENT_TIMESTAMP' on TEXT column → would output DEFAULT CURRENT_TIMESTAMP (unquoted - invalid SQL)
3. ❌ 'CURRENT_TIMESTAMP' on CHAR column → would output DEFAULT CURRENT_TIMESTAMP (unquoted - invalid SQL)
4. ❌ 'CURRENT_TIMESTAMP' on INTEGER column → would output DEFAULT CURRENT_TIMESTAMP (unquoted - invalid SQL)

The Fix:
Only skip quoting CURRENT_TIMESTAMP for datetime-related column types.
For other types (like string), it should be quoted as a literal string value.