Escaped placeholders are evaluated if the PropertySource has placeholder support

This is a follow-up of #34315

Consider the following example:

```properties
prop1=value1
prop2=value2\\${prop1}
```

`prop2` should evaluate to `value2${prop1}` as the placeholder has been escaped.

This works for basic `PropertySource` implementations, but if the source is backed by `PropertySourcesPlaceholderConfigurer` the escaped `${prop1}` is still evaluated even though it should not be.

