Remove experimental `constrain_avoidance` from `NavigationRegion2D` #27860

Job	Run time
📊 Static checks / Code style, file formatting, and docs	3m 29s
🍏 iOS / Template (target=template_release)	14m 54s
🍎 macOS / Editor (target=editor, tests=yes)	21m 6s
🤖 Android / Template (target=template_release)	22m 46s
🍎 macOS / Template (target=template_release)	16m 24s
🐧 Linux / Editor w/ Mono (target=editor)	5m 57s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	17m 22s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	30m 21s
🌐 Web / Template w/o threads (target=template_release, threads=no)	17m 34s
🏁 Windows / Editor (target=editor, tests=yes)	39m 10s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	27m 43s
🏁 Windows / Template (target=template_release)	24m 9s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	26m 22s
🐧 Linux / Template w/ Mono (target=template_release)	18m 30s
🐧 Linux / Minimal template (target=template_release, everything disabled)	13m 28s
🪲 Godot CPP / Build and test Godot CPP	7m 11s
	5h 6m 26s

Provide feedback