DEPR: `Styler.render` in favour of `Styler.to_html`
This PR deprecates the method `Styler.render()` in favour of `Styler.to_html()`, which has been recently introduced and produces the same result under the default case of no arguments.

Additionally this PR has added a test to a new file `pandas/tests/io/formats/style/test_deprecated.py`, and it has moved some of the previous Styler deprecation tests into this file.
