Fix DataSourceConfiguration isTestOnReturn()

Fix setTestOnReturn to use isTestOnReturn.
Create RelaxedPropertyResolver

Create RelaxedPropertyResolver class that can be used to get values
from another PropertyResolver (probably an Environment) using the
same relaxed rules as the RelaxedDataBinder.

The commit extracts the relaxed naming rules from RelaxedDataBinder
into a new RelaxedNames class.

Issue: #55621278
Use RelaxedPropertyResolver in auto-configuration

Update several existing auto-configuration classes to use the new
RelaxedPropertyResolver.

This commit also rename the spring.template property to spring.thymeleaf
in case we wish to support more templating engines in the future.
Rework database auto-configure

Rework several aspects of database auto-configuration:

- Use RelaxedPropertyResolver to obtain property values
- Extract EmbeddedDatabaseConnection from EmbeddedDatabaseConfiguration
- Rename several configuration classes for consistency

Issue: #53028397
Add sample yml to docs