Change behavior of lambdify deprecation

Now:
- only warns if `expr.is_Matrix`
- only warns once

Fixes #8030.
You can also fill and dedent it manually, like

```py
        SymPyDeprecationWarning("""\
Currently, `sympy.Matrix` is replaced with `numpy.matrix` if the NumPy
package is utilized in lambdify. In future versions of SymPy (>
0.7.6), we will default to replacing `sympy.Matrix` with
`numpy.array`. To use the future behavior now, supply the kwarg
`modules=[{'ImmutableMatrix': numpy.array}, 'numpy']`. The old
behavior can be retained in future versions by supplying
`modules=[{'ImmutableMatrix': numpy.matrix}, 'numpy']`.
""", issue 7853).warn()
```

If you do this you can avoid the awkward break after the `>`.  Although in that case, what is there is already preferable, as it avoids all awkward breaks.