emqx: depend on erlang@26

[paulo-ferraz-oliveira]

We're pining emqx to erlang@26 at this moment, since we're getting ready for erlang@27, not supported by the former.

Reasoning: it doesn't compile/work with OTP 27 yet.

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[paulo-ferraz-oliveira]

There's #183297 but it handles a different issue, so a rebase (here, or there) might be in order depending on which PR gets merged first.

[carlocab]

Thanks!

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