DOC: Adjust deprecation messages

[MartinThoma]

We are trying to break as few running systems as possible. For this
reason we keep the adapter methods / classes in PyPDF2 until 3.0.0.

This commit is done to not confuse people. It will not be backported
to the 1.x branch though.

[codecov]

Codecov Report

Merging #919 (f8e5d1a) into main (07bb859) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #919   +/-   ##
=======================================
  Coverage   77.81%   77.81%           
=======================================
  Files          16       16           
  Lines        4327     4327           
  Branches      812      812           
=======================================
  Hits         3367     3367           
  Misses        788      788           
  Partials      172      172           

Impacted Files	Coverage Δ
PyPDF2/_page.py	79.37% <ø> (ø)
PyPDF2/_reader.py	77.01% <ø> (ø)
PyPDF2/_writer.py	78.90% <ø> (ø)
PyPDF2/generic.py	77.77% <ø> (ø)
PyPDF2/xmp.py	52.06% <ø> (ø)
PyPDF2/__init__.py	100.00% <100.00%> (ø)
PyPDF2/_utils.py	89.65% <100.00%> (ø)

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