Adding warning about non-functioning NRAO module

[bsipocz]

I suggest putting on a 6-months duration for this warning and removing the module after that, unless the updates as per #2316 are done by then.

I don't add a changelog now but also leave off the label, the non-functioning of the module needs to be mentioned if this ends up in a release without the updates PR.

[codecov]

Codecov Report

Merging #2387 (d6dbc7c) into main (5ac1834) will decrease coverage by 0.45%.
The diff coverage is n/a.

❗ Current head d6dbc7c differs from pull request most recent head 3917092. Consider uploading reports for the commit 3917092 to get more accurate results

@@            Coverage Diff             @@
##             main    #2387      +/-   ##
==========================================
- Coverage   63.29%   62.84%   -0.46%     
==========================================
  Files         132      133       +1     
  Lines       17256    17270      +14     
==========================================
- Hits        10923    10853      -70     
- Misses       6333     6417      +84     

Impacted Files	Coverage Δ
astroquery/nrao/core.py	24.57% <ø> (-46.46%)	⬇️
astroquery/utils/cleanup_downloads.py	80.00% <0.00%> (ø)

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[keflavich]

just a minor language nitpick; 👍 otherwise