Sync CI configuration into 1.5.x branch

[awaelchli]

What does this PR do?

Syncs the CI configuration from master, dropping the support for torch 1.6 and fixing our CI jobs here.
All changes in these files are now identical to the ones on master, no other changes were done here.

Done via git checkout master .github/workflows requirements/ ... etc.

These are the PRs that updated the configuration on master:
#10367
#10075
#10088
#10390

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• 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? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @carmocca @akihironitta @Borda

[carmocca]

Looks good, tests are green.

Remember to update the CHANGELOG: https://github.com/PyTorchLightning/pytorch-lightning/blame/master/CHANGELOG.md#L130

[akihironitta]

LGTM :]

[tchaton]

LGTM !