Update schedule.json to reduce v26.x Maintenance

[aduh95]

We've discussed in the Release WG meeting about how having long maintenance put a too heavy burden for security releases. One proposal was to reduce the Maintenance window so we don't end up in a situation where we have 4 non-EoL release lines to maintain (as it is the case currently, with up to 5 even for a brief period of the year).

I don't know if we want to do that for every LTS lines, or rather every two years (i.e. 26.x 2.5 years of support, 28.x 3 years, 30.x 2.5 years, 32.x 3 years, etc.), and it's obviously hard to predict where the project will be in 2029, but it's probably better to discuss it now so consumers can adapt.

wdyt?