CI: Don't move to Lightning Trainer / PL 1.7.0 (temporary fix)

[krshrimali]

What does this PR do?

PL 1.7.0 broke the CI (~14 failures), partially because of BC changes and sometimes because of changes in PL 1.7.0. This blocks us from merging PRs, community contributions, and more.

Most of the failures have been fixed, but there are a couple of them remaining and are being addressed in #1410, and other relevant issues in the Lightning repository. We will go back to supporting PL's latest versions once issues are fixed. (partially addresses #1413)

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

cc: @Borda @ethanwharris @carmocca (for visibility)

[codecov]

Codecov Report

Merging #1415 (3bca79b) into master (8737ee8) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1415   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files         286      286           
  Lines       12857    12857           
=======================================
+ Hits        11799    11800    +1     
+ Misses       1058     1057    -1     

Flag	Coverage Δ
unittests	91.77% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/text/question_answering/model.py	94.55% <0.00%> (+0.68%)	⬆️

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[ethanwharris]

Closing this as addressed in #1410