Bump ubuntu from `53958ec` to `b7f4819` in /ptf #923

Job	Run time
build-ptf / docker-build	1m 21s
build / docker-build	1m 46s
check-spelling / check-spelling	14s
route-control-tests / route-control-tests	19s
fossa-scan / fossa-scan	41s
static-analysis / staticcheck	26s
hadolint / hadolint	7s
integration-tests-bess / unit-tests	1m 2s
analysis / score-analysis	24s
license-check / license-check	9s
unit-tests-pfcp / unit-tests	1m 4s
lint / lint	16s
	7m 49s

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