[#230,irods/irods#8351,irods/irods#8877] Remove Ubuntu 20.04 and EL8, Add Ubuntu 26.04

[SwooshyCueb]

Addresses #230
In service of irods/irods#8351
In service of irods/irods#8877

[korydraughn]

Split the Remove old distro support commit into two separate commits. One for ub20 and another for el8.

That aligns with our EOL tagging scheme for other distros.

[SwooshyCueb]

We can just add two tags to the same commit, can't we?

[korydraughn]

Yes, we can add multiple tags. Having separate commits just makes it easier to see what changed later. Not a blocker.

Thinking about it a little more, I don't think we should remove those distros just yet. Plugins for 4.3 are still supported and may need to be released. The 4.3 server is the only component that will not receive another release.

So, pulling that commit into a separate PR until the 4.3 plugins are EOL seems like a good step.

Also, let's update the commit message to mention ub20 and el8 instead of old distros.