update release plan for 7.1.8 and 7.2.4

[Stéphane Guillou]

Hi all

Could someone who has the rights to do so please update the main release 
plan page, since there were unplanned security fix releases for 7.1 and 7.2?

The detailed release plans for both 7.1 and 7.2 have already been 
updated accordingly, but not the overview: 
https://wiki.documentfoundation.org/ReleasePlan

Cheers!

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Stéphane Guillou
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