Version compatibility chart in readme is out of date

[deepthi]

Right now we have operator v2.6.* with vitess v12.0.* and v2.7.* also with vitess v12.0.*. We should change it to

v2.6.* - Vitess v12.0.* and v13.0.* v2.7.* - Vitess v14.0.*

It is worth reviewing the whole document to see if anything else needs to be changed.

[Verolop]

I think @frouioui updated this here: https://github.com/planetscale/vitess-operator/commit/1e480c73a9788f907df52d7ab2ca62c7d2c0b1fc I added Vitess v.12.0.* for v2.6.* https://github.com/planetscale/vitess-operator/pull/269