sagemathgh-40421: Limit the use of RingExtensions in Drinfeld modules
    
Behaviour before this PR:
If `phi` is a Drinfeld module, the method `phi.ore_polring()` used to
return an Ore polynomial ring with coefficients in `RingExtension`.

This causes a lot of headache because currently an instance of
`RingExtension`  does not always belong to the appropriate category and,
consequently, does not implement all relevant methods and/or is not
recognized as what it actually is (e.g. a finite field).

In this PR, we propose a simple fix: we let `phi.ore_polring()` return
an Ore polynomial ring over the underlying regular field. This only
changes the way things are printed, but does not alter the user
interface.

We will maybe come back later to the original implementation when
`RingExtension` will be fixed (which does not seem to be
straightforward).

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.
    
URL: sagemath#40421
Reported by: Xavier Caruso
Reviewer(s): Antoine Leudière

sagemathgh-40430: Use the variable name τ instead of t for Drinfeld modules
    
In the theory of Drinfeld module, the classical name of the
noncommutative variable is `τ`. However, in Sage, we currently use `t`,
which might conflict with other standard notations (often the variable
name of the underlying function ring is denoted by `t`).
In this PR, we propose to shift to the notation `τ`. It just changes how
the outputs are printed (by default), not the interface.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

sagemath#40420, sagemath#40421
    
URL: sagemath#40430
Reported by: Xavier Caruso
Reviewer(s): Antoine Leudière