Change behavior of lambdify deprecation

Now:
- only warns if `expr.is_Matrix`
- only warns once

Fixes #8030.
btw, this is a good time to use filldedent:

```Warning(filldedent('''
            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']`.
            '''),
            issues=7853).warn()
```

In this case, the message will look like

```
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']`.
```