On the Downloads page, distinguish between the Git tag date and the official release announcement date

[DilumAluthge]

Users often confuse the date of the Git tag with the date of the official release announcement. For a recent example, see this Discourse thread. However, I have seen this same confusion multiple times in the past.

Therefore, on the downloads page, we should explicitly distinguish between the Git tag date and the official release announcement date.

[github-actions[bot]]

Once the build has completed, you can preview your PR at this URL: https://julialang.netlify.app/previews/PR1750/ in ~15 minutes

[ararslan]

Rather than manually tracking two separate dates, could we just have a prominent note near the top of the downloads page that explains what the dates listed with the versions are? I'm not sure that the announced date is all that useful in practice.