User warning: pgAdmin v6.6 startup time on macOS

Unfortunately we've since discovered an issue affecting some installations on macOS where startup of the application can take over a minute. We believe this is caused by this issue in NWjs: https://github.com/nwjs/nw.js/issues/7851

Once started, pgAdmin works as it should, based on our testing.

Despite this being an annoyance rather than a loss of functionality, we've decided we will release v6.7 as soon as possible, in the next few days. 

If you are a macOS user, you may wish to wait for the 6.7 release instead of upgrading to 6.6, in case you are one of the unlucky users that will be affected.
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