Set C++20 as standard in CMake project

Adjust to C++20 (#636)

* Update GitHub Actions Linux compilers

* Update MSVC project to use C++20

* Update XCode project to use C++20

* Satisfy new C++20 requirement of total equality

* Explicitly type as wxString

Otherwise MSVC gets confused.

Update version of checkout action (#632)

Old versions are deprecated, see <https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/>

Update version of checkout action (#632)

Old versions are deprecated, see <https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/>

Update version of checkout action (#632)

Old versions are deprecated, see <https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/>

Revert wxAUI enhancements (#628)

wxAUI does not properly support the recent Windows Monitor Awareness modes

Change multiline string literals `xgettext` misses (#627)

* Change multiline string literals `xgettext` misses

Fixed as described in the [wikidump of the Audacity Wiki as of March 31, 2023 that can be found in the Internet Archive](https://web.archive.org/web/20221112235157/https://wiki.audacityteam.org/wiki/Translating_Audacity/Writing_Translatable_Code#Dealing_with_Long_Strings).

* pt_BR: update translation

* Update pt_BR keyboard accelerators

Offer option to reset toolbar

Also trigger an update at the wxAUIManager on DPI change

Use plain background

Fix zen mode