Fix invalid escape sequence warnings #11233

Job	Run time
htmldocs	25m 10s
package-arch (debian:bullseye)	29m 6s
cppcheck	2m 54s
package-arch (debian:bookworm)	29m 39s
package-indep (debian:bullseye)	42m 57s
package-arch (debian:sid)	31m 30s
package-indep (debian:bookworm)	33m 33s
rip-and-test	26m 30s
rip-and-test-clang	29m 47s
package-indep (debian:sid)	38m 50s
	4h 49m 56s

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