julia: introduce LTS and stable aliases
julia: introduce LTS and stable aliases

According to the Julia release process [1] there will only ever be two
supported release branches at any given time: stable and LTS. Once a new
version of either is released, support for the previous release will be
dropped upstream. Introducing aliases for these branches allows our
users to easily track either depending on their need for stability.

[1]: https://julialang.org/blog/2019/08/release-process/#long_term_support

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