depends: Update Qt to 5.15.5

[alamshafil]

This PR updates the Qt dependency to 5.15.5 and contains the necessary changes for it.
Qt 5.15.5 was chosen because it is a stable LTS release and is the same version used in Bitcoin.

The way I made this PR was cherry-picking each Bitcoin QT update starting from the version Dogecoin uses. Once I got to the final update, I separated the resulting diff into separate commits.

I have left both the ceb93fa and 6ad6276 commits under "[WIP]" because the Qt commit contains many changes that was merged into one. I also am not sure if the 6ad6276 commit was done properly since Bitcoin had completely reworked the file.

I am marking this PR as a draft because it contains many dependency updates such as OpenSSL that can be done seperatly and should be tested. Regrading OpenSSL, Qt 5.15.5 requires a min version of 1.1.1 and I picked 1.1.1s which was the latest at the time.

[patricklodder]

I've added the qt tarballs to the cache on depends.dogecoincore.org so that it won't fail on download and have restarted the CI. Hopefully we'll get a complete list of everything that needs to be looked at re: backward compatibility.

[patricklodder]

Re: missing patch files in CI run. This is a pretty annoying "feature" of a strategy where we're rejecting *.patch in .gitignore.

I've added an exception list with #3292 where depends/patches/**/*.patch is allowed, so that this becomes less of a pain in the behind.

[alamshafil]

Looks like I got most of the CI to pass with a couple of failures.

x86_64-linux-nowallet Looks like Qt 5.15.5 breaks glibc compatibility (I found some features to disable in configure that could fix)

x86_64-macos
Qt 5.15.5 includes <os/log.h> which is only found on >=10.12. Looking further into it, it is trival to get Qt 5.15.5 to work on =10.11, since it requires syscalls only found on >=10.13.

x86_64-win-experimental
Is this failing because of bitcoin_qt.m4? Need to properly fix that file I believe.

[alamshafil]

@patricklodder

Since Qt 5.15.5 will not support macOS 10.11, should we try to get Qt 5.11.x working (supports 10.11) on 1.14.x or just move this to 1.21?

[patricklodder]

Since Qt 5.15.5 will not support macOS 10.11, should we try to get Qt 5.11.x working [..]?

That could work, what benefits do we get, and which benefits of 5.15 do we not get?

I'd also recommend to do that without making changes to the symbol_check scripts, because these safeguard backward compatibility. Anything that breaks compatibility will have to go into 1.21, I commented on that, and alternatives, a while ago:

From a lifecycle management p.o.v., [breaking compatibility] can be solved by:

1. Waiting for 1.21, or
2. Do an intermediate major release (1.15?) where we retain 1.14 maintenance for a while, but do all feature development on a newer version that (across the board) has different backward compatibility targets.

I'd suggest opening a separate issue for this. If I hear nothing, I will default to option 1 and put all the eggs in the 1.21 basket.

[alamshafil]

Since this will break backward compatibility, I will close this PR.