Fix Horn-Schunck depreciated namespace

[tianyizheng02]

Describe your change:

Updated the namespace of convolve in computer_vision/horn_schunck.py from scipy.ndimage.filters to scipy.ndimage, as the former is depreciated (see the release notes for SciPy release 1.8.0 for more details).

Fixes #7299

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
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[tianyizheng02]

@CaedenPH @cclauss Could one of you take a look to see if this solves the issue and is good for merging?

[cclauss]

Please provide us a url to the docs that discuss the deprecation.

[tianyizheng02]

@cclauss The release notes for SciPy 1.8.0 say the following:

In this release, all namespaces that were private but happened to miss underscores in their names have been deprecated. These include (as examples, there are many more):
- scipy.signal.spline
- scipy.ndimage.filters
...
All functions and other objects in these namespaces that were meant to be public are accessible from their respective public namespace