docs(*) upgrade guide for 1.2.1

[jakubdyszkiewicz]

Leaving the note that during the multizone upgrade ZoneIngress will be offline, which does not affect the cross-zone traffic.
We are not releasing the version with v this time so removing this entry from the changelog.

[codecov-commenter]

Codecov Report

Merging #2260 (5eb9d40) into master (6dd1ae1) will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2260      +/-   ##
==========================================
+ Coverage   51.65%   51.99%   +0.33%     
==========================================
  Files         910      914       +4     
  Lines       41098    41134      +36     
==========================================
+ Hits        21229    21386     +157     
+ Misses      17833    17682     -151     
- Partials     2036     2066      +30     

Impacted Files	Coverage Δ
pkg/kds/client/sink.go	43.58% <0.00%> (-5.13%)	⬇️
...kg/plugins/resources/k8s/native/pkg/test/within.go	100.00% <0.00%> (ø)
api/internal/util/proto/proto.go	62.50% <0.00%> (ø)
...k8s/native/controllers/proxytemplate_controller.go	0.00% <0.00%> (ø)
api/internal/util/proto/types.go	50.00% <0.00%> (ø)
pkg/xds/generator/direct_access_proxy_generator.go	81.57% <0.00%> (+1.31%)	⬆️
api/mesh/v1alpha1/dataplane_helpers.go	77.84% <0.00%> (+1.70%)	⬆️
pkg/core/resources/manager/cache.go	83.09% <0.00%> (+2.81%)	⬆️
pkg/mads/server/server.go	84.76% <0.00%> (+2.85%)	⬆️
api/mesh/v1alpha1/dataplane_insight_helpers.go	80.24% <0.00%> (+4.93%)	⬆️
... and 14 more

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