CI: Minor update of build scripts and GitHub Actions workflows and actions

[PatTheMav]

Description

Updates the following aspects of the current build scripts and GitHub Actions workflows and actions:

• Build scripts on Linux require Ubuntu 24.04 and x86_64 - any preliminary/experimental support for aarch64 was removed
• Qt6 is used by Ubuntu builds by defaults, associated build spec settings are thus superfluous and have been removed
• Build scripts on macOS and Linux require Zsh 5.9
• macOS builds use Xcode 15.4 by default
• Experimental support for CCache on Windows was removed
• qt-xml-validator now uses the check-changes action to check for and get a list of changed UI files
• Several complex shell script snippets have been documented in-line
• XSLT stylesheets used by sparkle-appcast action have been updated with doc comments to explain how they work (XSLT and XPATH are remnants of a past age of Java, SOAP, and XHTML, so not many might be familiar with these)
• sphinx-publish-action has been updated to use the precise commit hash of version 1.2.0
• wrangler-action has been updated to commit hash of version 3.7.0

Motivation and Context

Regular maintenance of CI workflows and build scripts.

How Has This Been Tested?

Will need some actual CI runs to test.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

The changes to pinned third-party actions look fine. The changes to Zsh and Ubuntu version requirements look fine. The documentation updates are largely fine. I don't know if anyone will have opinions about the other changes, but I'll let it sit at least overnight.

[Vainock]

If you are on it, could you take a look at why

obs-studio/.github/workflows/scheduled.yaml

Line 135 in 61d74fb

if: fromJSON(steps.checks.outputs.hasChangedFiles)

causes this failure? https://github.com/obsproject/obs-studio/actions/runs/10875623224/job/30174384982

This line was changed by #11253.
Rather than not triggering the upload action, it now causes this issue.

[PatTheMav]

If you are on it, could you take a look at why

obs-studio/.github/workflows/scheduled.yaml

Line 135 in 61d74fb

if: fromJSON(steps.checks.outputs.hasChangedFiles)

causes this failure? https://github.com/obsproject/obs-studio/actions/runs/10875623224/job/30174384982
This line was changed by #11253. Rather than not triggering the upload action, it now causes this issue.

That's because the check never ran, so there is no steps.check and thus the expression probably yields null and it cannot be used with fromJSON.

In theory we could use if: steps.checks.outputs.hasChangedFiles && fromJSON(steps.checks.outputs.hasChangedFiles), if the runtime short-circuits the condition:

• If the step hasn't run, then the value itself should be null, which is interpreted as false and the check short-circuits
• If the step has run, the value itself will be non-null, so it's "truthy" and the fromJSON check converts only the string "true" into the actual boolean true.