Lightning 1.5 release

[awaelchli]

What does this PR do?

Updates the changelog and several attributes in preparation of the 1.5 release.

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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)

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[tchaton]

LGTM !

[Borda]

lgtm 🐰

[codecov]

Codecov Report

Merging #10306 (76cb77e) into master (373c32e) will not change coverage.
The diff coverage is 100%.

❗ Current head 76cb77e differs from pull request most recent head f2b81f0. Consider uploading reports for the commit f2b81f0 to get more accurate results

@@           Coverage Diff           @@
##           master   #10306   +/-   ##
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  Coverage      93%      93%           
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  Files         182      182           
  Lines       16487    16487           
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  Hits        15256    15256           
  Misses       1231     1231           

[akihironitta]

Update "1.4.x" to "1.5.x" here or in another PR after the release?

https://github.com/PyTorchLightning/pytorch-lightning/blob/9e844d9db6be8e597ab468550234baf5216f6a93/README.md?plain=1#L127