Add builds for qt5.15lts-21.08

[aleixpol]

Since Qt is not releasing anymore, we are still maintaining the one based on their last release. Since there's a new SDK release, we decided to create a new version of ours based on it. The naming is a bit wonky for sure, but at least we get to use the last versions of our base.

[aleixpol]

I don't have merging powers (or I just can't see a Merge button). Can we please get this merged? Thanks!

[tinywrkb]

/ping @barthalion @nedrichards

[barthalion]

Started new build: https://flathub.org/builds/#/builders/32/builds/60571

[travier]

Failed on:

-- The following REQUIRED packages have not been found:
 * PlasmaWaylandProtocols (required version >= 1.4.0)
CMake Error at /usr/share/cmake-3.21/Modules/FeatureSummary.cmake:464 (message):
  feature_summary() Error: REQUIRED package(s) are missing, aborting CMake
  run.
Call Stack (most recent call first):
  CMakeLists.txt:109 (feature_summary)

[travier]

https://invent.kde.org/packaging/flatpak-kde-runtime/-/merge_requests/58

[flathubbot]

Ignoring bot build request due to repo lookup error: Invalid id buildbot-config.

[flathubbot]

Ignoring bot build request due to repo lookup error: Invalid id buildbot-config.

[flathubbot]

Queued test build for org.kde.Sdk.

[flathubbot]

Started test build 60756

[flathubbot]

Queued test build for org.kde.Sdk.

[flathubbot]

Build 60756 failed

[flathubbot]

Started test build 60757

[flathubbot]

Build 60757 failed

[barthalion]

@MehdiChinoune Would be much appreciated if you don't try to trigger new builds here. :)

New build is at https://flathub.org/builds/#/builders/32/builds/60758.