chore(xds): don't set deprecated dns_resolver_config

[michaelbeaumont]

Checklist prior to review

• Link to docs PR or issue -- none
• Link to UI issue or PR -- none
• Is the issue worked on linked? -- none
• Unit Tests -- none
• E2E Tests -- none
• Manual Universal Tests -- none
• Manual Kubernetes Tests -- none
• Do you need to update UPGRADE.md? -- no
• Does it need to be backported according to the backporting policy? -- no

[codecov-commenter]

Codecov Report

Merging #4702 (f0b453b) into master (d841111) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4702   +/-   ##
=======================================
  Coverage   46.42%   46.43%           
=======================================
  Files         687      687           
  Lines       46692    46695    +3     
=======================================
+ Hits        21677    21683    +6     
+ Misses      23094    23092    -2     
+ Partials     1921     1920    -1     

Impacted Files	Coverage Δ
pkg/xds/envoy/listeners/v3/dns_configurer.go	94.33% <100.00%> (+0.16%)	⬆️
pkg/core/resources/manager/cache.go	85.71% <0.00%> (ø)
pkg/plugins/leader/postgres/leader_elector.go	89.36% <0.00%> (+6.38%)	⬆️

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