CI: Update FreeBSD image version for Cirrus runner. #3321

Job	Run time
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 13s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 38s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 36s
cmake-offline-googletest	4m 15s
cmake-offline-googletest-src	3m 55s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 17s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 45s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 6s
system-sexpp, sexpp shared libs on, rnp shared libs on	3m 13s
system-sexpp, sexpp shared libs on, rnp shared libs off	2m 54s
system-sexpp, sexpp shared libs off, rnp shared libs on	2m 46s
package-source	34s
system-sexpp, sexpp shared libs off, rnp shared libs off	2m 57s
package	51s
debian-tests	11s
	47m 11s

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