build: remove buster, replace with bullseye when necessary

[aminvakil]

It's been three years since buster does not gets updated from Debian Security Team and it's been a year since Debian LTS Team does not support that either.

Now that trixie has been added as well to this repository, I've removed all buster tests.

There were also some tests which only were tested on buster, I've changed them to bullseye for now to expect minimum changes, and I'm going to change them to trixie with is Debian stable now.

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fluent/fluent-bit-docs#2068

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[patrick-stephens]

I would disable the build but not remove the option for folks to build, I know it's eol but that's not the same as no one uses it (look at Centos). We can just remove it from all the CI but leave the container definition I think.

[aminvakil]

I would disable the build but not remove the option for folks to build, I know it's eol but that's not the same as no one uses it (look at Centos). We can just remove it from all the CI but leave the container definition I think.

65c15b4

Does this suffice?