p-adic __getitem__ should ressemble behaviour of number fields

[saraedum]

For a number field element, x[i] returns the coefficient of x when written as a polynomial in the generator of the number field.

For p-adics, it currently returns the coefficient in the π-adic expansion.

A deprecation warning has been introduced in #14825 to change the behaviour of p-adic elements.

See also #26408, #13300.

Depends on #14825

Component: padics

Keywords: padicBordeaux

Issue created by migration from https://trac.sagemath.org/ticket/26406

[roed314]

Changed keywords from none to padicBordeaux