Address deprecation of Imagick::radialBlurImage

[JoelAlphonso]

Originally posted by @mcaskill in #55 (comment)

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Add note in this method's block comment and the README about the deprecation of Imagick::radialBlurImage and replacement with Imagick::rotationalBlurImage.

We should ensure this change is surfaced in this pull request's summary and the repository's CHANGELOG.

This commit is more than just a fix, it's potentially a breaking change since it requires a specific version of software/extension.

We should rename radial to rotational as well and add a radial alias with a deprecation notice. Otherwise, open an issue or draft pull request to address this change later along with ImageMagick equivalent.

This may be an issue in case servers or local environments with an out-of-date Imagick extension.

FYI @mducharme