[2.3.x Backport] liveness probe to not use URI (#8128)

[tybritten]

Switches pg bouncer liveness probe to not use URI

[jrockway]

Very nice!

[codecov]

Codecov Report

Merging #8155 (42c0ed1) into 2.3.x (a9b7db4) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            2.3.x    #8155      +/-   ##
==========================================
- Coverage   22.29%   22.25%   -0.05%     
==========================================
  Files         415      415              
  Lines      112189   112189              
==========================================
- Hits        25013    24967      -46     
- Misses      82904    82933      +29     
- Partials     4272     4289      +17     

Impacted Files	Coverage Δ
src/internal/dlock/dlock.go	70.83% <0.00%> (-8.34%)	⬇️
src/server/pfs/server/master.go	75.64% <0.00%> (-6.42%)	⬇️
src/internal/storage/fileset/util.go	77.01% <0.00%> (-3.45%)	⬇️
src/internal/task/util.go	72.05% <0.00%> (-2.95%)	⬇️
src/internal/transactionenv/env.go	61.11% <0.00%> (-2.78%)	⬇️
src/internal/stream/priority_queue.go	92.68% <0.00%> (-2.44%)	⬇️
src/server/pfs/server/trigger.go	76.78% <0.00%> (-1.79%)	⬇️
src/server/pfs/server/compaction.go	69.06% <0.00%> (-1.66%)	⬇️
src/internal/task/etcd_service.go	80.70% <0.00%> (-1.58%)	⬇️
src/internal/storage/fileset/postgres_store.go	83.67% <0.00%> (-1.03%)	⬇️
... and 9 more

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