Fix invalid escape sequence warnings #11213

Job	Run time
htmldocs	25m 18s
cppcheck	2m 59s
package-indep (debian:bullseye)	44m 50s
package-arch (debian:bullseye)	29m 25s
package-indep (debian:bookworm)	33m 24s
package-arch (debian:bookworm)	29m 35s
package-indep (debian:sid)	39m 42s
rip-and-test	26m 22s
package-arch (debian:sid)	31m 31s
rip-and-test-clang	27m 37s
	4h 50m 43s

Provide feedback