Auto merge of #29916 - mrobinson:rename-meta-directories, r=atbrakhi

Rename metadata directories

Rename metadata directories and update all references. This is in preparation
for making Layout 2020 the default layout system.

This does the following renames:
 - `tests/wpt/metadata` → `tests/wpt/meta-legacy-layout`
 - `tests/wpt/metadata-layout-2020` → `tests/wpt/meta`
 - `tests/wpt/mozilla/meta` → `tests/wpt/mozilla/meta-legacy-layout`
 - `tests/wpt/mozilla/meta-layout-2020` → `tests/wpt/mozilla/meta`

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