Add 8.4 (LTS) variant

[jnoordsij]

This PR adds 8.4, the new LTS release, as variant. See also https://dev.mysql.com/doc/relnotes/mysql/8.4/en/news-8-4-0.html.

To do so, the update.sh script has been updated to use "https://repo.mysql.com/yum/mysql-tools-$version-community/el/$oracleVersion" as new default url for tools, hoping this is the new convention (didn't check for any official statements); see also https://repo.mysql.com/yum/.

No Debian variant was added per #867 (comment).

Moreover, I've taken the liberty to add an lts alias, which seems appropriate to me. If not, feel free to (ask me to) remove that.

Finally, when going through the sources I noticed that Oracle Linux 9 has been released a while back. I could not find any support policies regarding OS versions for this image so quickly, but if variants are preferably kept on the same OS for stability, considering to use v9 for this variant is probably appropriate. Otherwise, updating could be left as a separate (unrelated) follow-up PR.

[yosifkit]

🤔 https://dev.mysql.com/blog-archive/introducing-mysql-innovation-and-long-term-support-lts-versions/
The next innovation release will be the first of the 9.x series and they are roughly quarterly, but not at the same time as an LTS. So, I think we should figure out how to disable innovation until we have 9.x (or maybe the innovation tags should point at the LTS?)

[tianon]

Hmm, yeah -- I think we probably shouldn't tag this new release as innovation, since it isn't technically in the innovation "track" (the latest "innovation" release is still 8.3).

I think at most we should update to avoid building innovation while any explicit release has a higher version number so that we don't have to revisit this once innovation gets 9.x -- we've got some code for that elsewhere that I could adapt here. 👍

As usual @jnoordsij, excellent and well-thought-out contribution. 😄 Thank you! ❤️

[tianon]

Oh sorry, I also think the lts alias is probably OK? It's clearly a word upstream is using to describe this. 👍

[tianon]

Oh and further, regarding upgrades like OL9, if upstream supports it (and recommends using it), we're generally in favor of doing it. Last time I looked, they did not have the Docker-specific package builds we're using available for it, but maybe that's changed, or is different for the new 8.4?