Remove dom extension after yiisoft/error-handler release

[samdark]

Once yiisoft/error-handler releases the change from yiisoft/error-handler#176, update YiiPress to that released version and remove ext-dom from static package builds.

Context:

• yiisoft/error-handler no longer requires ext-dom on master after Fix #176: Remove unused ext-dom dependency yiisoft/error-handler#176.
• YiiPress currently still keeps dom because the released yiisoft/error-handler version still requires it.
• A temporary measurement with yiisoft/error-handler dev-master and dom removed produced a Linux binary of 6,119,026 bytes versus the current no-xml baseline of 6,222,964 bytes.
• Measured saving: 103,938 bytes post-UPX, about 1.67%.
• libxml2 remains because xmlwriter is still required, so this only removes the DOM extension code.

Tasks:

• Wait for a yiisoft/error-handler release containing Fix #176: Remove unused ext-dom dependency yiisoft/error-handler#176.
• Update composer.json/composer.lock to the released version, not dev-master.
• Remove dom from docker/Dockerfile and platform packaging scripts.
• Update packaging configuration tests.
• Build Linux package and re-measure size.