reduce requirement to 2-core once they up the RAM

[CSchank]

GitHub is upping the amount of RAM for their 2-core containers from 4gb to 8gb (https://github.blog/changelog/2023-08-04-codespaces-gives-you-a-free-upgrade/). Previously we ran into issues pretty quickly with only 4gb, so we needed to use the 4-core machines by default. But now that 2-core machines will have double the RAM, we'll be able to go back down to the 2-core ones by default and give people double the monthly time. People can still opt for the 4-core, 16gb one if they need. We'll make this change once the 2-core 8gb update goes live (apparently in the next few days).