Pin to PL 0.29

[lillian542]

Description of the Change:

• The plugin's setup.py pins to a maximum version of PennyLane 0.29
• The readme has a callout box announcing that the plugin is no longer supported and compatible with max 0.29. The docs page has a similar callout. (These both seem to draw from the same part of README.rst)

[codecov]

Codecov Report

Merging #134 (87048ec) into master (19285bc) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #134   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          498       498           
=========================================
  Hits           498       498           

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[lillian542]

[sc-35008]

[mudit2812]

Thanks for the change 🚀

[timmysilv]

Some extra spots I'd change:

• The "Dependencies" section of the README should also mention the max version
• requirements-ci.txt needs a maximum version or else tests will fail once 0.30 is released