MNT: Deprecate DataobjImage.get_data, schedule for removal in 5.0

[effigies]

Been threatening this for a while, and are over a year behind schedule.

Closes #770.

[codecov]

Codecov Report

Merging #794 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #794      +/-   ##
=========================================
+ Coverage    90.1%   90.2%   +0.09%     
=========================================
  Files          95      95              
  Lines       11879   12072     +193     
  Branches     2118    2181      +63     
=========================================
+ Hits        10704   10889     +185     
- Misses        834     837       +3     
- Partials      341     346       +5

Impacted Files	Coverage Δ
nibabel/dataobj_images.py	95.08% <100%> (+0.08%)	⬆️
nibabel/pydicom_compat.py	89.28% <0%> (-10.72%)	⬇️
nibabel/nicom/dicomwrappers.py	92.72% <0%> (+1.19%)	⬆️
nibabel/openers.py	94.89% <0%> (+4.08%)	⬆️

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