Update Supported Platforms

[HanaGemela]

• Announce depreciation for the next release on central

• Add changed platforms to Release Notes - no release notes for 2.6.3

• Update Desktop client download page

• Update client docs (update master and backport to 2.6) to display supported platforms for the latest stable release.
  Master: Update installing.adoc #7929 Update installing.adoc with currently supported platforms #7915
  2.6: Update documentation #7974

• Update server docs (update master and backport to 10.4) to display supported platforms for the latest stable release
  Master: Update system_requirements.adoc docs#2712
  10.5: [10.5] Update system_requirements.adoc docs#2713
  10.4: [10.4] Update system_requirements.adoc docs#2714

Update OBS:

• deepcopy current client including its Qt512x dependencies, so that platforms dont get lost for the current one.
• apply platform change to isv:ownCloud:Qt512x
• do the same in obs.int / drone toolchain

[HanaGemela]

@michaelstingl Do we have release notes?

[jnweiger]

Checkmarked 'apply platform change to isv:ownCloud:Qt512x' without doing anything.
Qt5127 had received all current platforms before the builds. I cannot think of anything to be done after a release.

Same difficulty with obs.int/drone: checkmarked without doing anything.

[HanaGemela]

All tasks done