CLN: enforce deprecation of the `method` keyword on `df.fillna`
xref #53496

enforced deprecation of the `method` keyword on `df.fillna/ser.fillna`
I enforced deprecation of the `method` keyword on `df.fillna/ser.fillna`. We deprecated the `method` keyword on `EA.fillna` as well. Is it OK if I enforce this deprecation in separate PR?

@jbrockmendel, could you please take a look at this PR? I think CI failures are unrelated to my changes.

 Thanks @natmokval  Thanks @mroeschke for reviewing this PR. 