cli,server: increase `GOGC` by default to 300%, add CLI flag

[nvb]

Closes #115164.

This commit introduces a new --go-gc-percent flag to CLI start command which controls the garbage collection target percentage on the Go runtime. The default value for the flag if neither the --go-gc-percent flag nor the GOGC env var is set is 300%.

To avoid introducing new OOMs, we only switch to this new default (up from 100%) if a soft memory limit is set, either manually or automatically.

Release note (cli change): A new flag --go-gc-percent is introduced to start command. It controls the garbage collection target percentage on the Go runtime, mirroring the existing GOGC environment variable. A garbage collection is triggered when the ratio of freshly allocated data to live data remaining after the previous collection reaches this percentage. If left unspecified, defaults to 300%.

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NOTE: leaving this in a draft state until I can run some benchmarks on this to confirm that it is a net positive.

[cockroach-teamcity]

This change is 

[nvb]

TFTR!

bors r+

[craig[bot]]

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