Update browser support to match for at least 1 year

Our current `.browserslistrc` currently sets support to `chrome 55` and `firefox 54`, which were initially released in 2016 and 2017 respectively. It's somewhat unreasonable to support almost 8+ year old browsers, especially considering most browser vendors push auto updates on a semi-regular basis.

We should instead move to a rolling model of setting our minimum browser support to only officially include browsers released within the past year or above a minimum threshhold, updating our `.browserslistrc` to the following:

> \> 1% or last 1 year
> last 2 safari versions

Unofficially, we can make this the new policy going forward but cannot fully implement the change for `.browserslistrc` until the [deprecation period](https://github.com/dequelabs/cauldron/blob/develop/CONTRIBUTING.md#deprecating) has passed.