[8.0] Update dirac_cert_convert.py, adding cmd-line option "--legacy" to allow pcks12 certificate with legacy crypto #11195

Job	Run time
check (pytest --runslow -k 'not test_BaseType_Unicode and not test_nestedStructure')	12m 25s
check-py39 (pytest --runslow -k 'not test_BaseType_Unicode and not test_nestedStructure')	12m 40s
check (pylint -j 0 -E src/)	2m 57s
check-py39 (pylint -j 0 -E src/)	2m 48s
check (mypy)	1m 31s
pre-commit	1m 4s
pycodestyle	7s
shellcheck	4s
Pylint for Python 2.7 in Pilot files	56s
	34m 32s

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