chore: reduce the max instructions of heartbeats from 4B to 2B #1205

Job	Run time
canister-build-reproducibility	3m 26s
cargo-clippy	1m 27s
cargo-build (ubuntu-20.04)	4m 2s
cargo-build (macos-12)	6m 47s
rustfmt	19s
ShellCheck	8s
benchmark	4m 28s
cycles_burn	2m 9s
charge-cycles-on-reject	2m 41s
cargo-tests (ubuntu-20.04)	3m 46s
cargo-tests (macos-12)	3m 31s
e2e-disable-api-if-not-fully-synced-flag	2m 56s
e2e-scenario-1	2m 48s
e2e-scenario-2	2m 26s
e2e-scenario-3	2m 27s
set_config	2m 10s
upgradability	2m 24s
watchdog_get_config	2m 35s
watchdog_health_status	2m 47s
watchdog_upgradability	2m 55s
watchdog_metrics	2m 37s
checks-pass	1s
	58m 50s

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