gh-36394: Deprecate the backslash operator
Deprecate the backslash operator.
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
Deprecate the backslash operator: it is underused and its current
implementation conflicts with standard Python usage of `\`. A [poll on
sage-devel ](https://groups.google.com/g/sage-
devel/c/8G-tPMdtNX4/m/tRCfUVHiCAAJ) supports this decision.
<!-- Describe your changes here in detail -->
The backslash operator is only used in a few places, mainly matrices and
matroids; those uses are deprecated.
<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->
- [X] The title is concise, informative, and self-explanatory.
- [X] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [X] I have created tests covering the changes.
- [X] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/36394
Reported by: John H. Palmieri
Reviewer(s): John H. Palmieri, Kwankyu Lee
