docs: drop V2_ROADMAP.md references from migration guide #192

Job	Run time
MSRV build (1.95)	32s
autobahn websocket suite	1m 30s
rustdoc	27s
fuzz harness compile check	1m 23s
coverage	53s
clippy (default)	22s
miri (tako-core + tako-extractors)	51s
clippy (tokio-rich)	31s
test (ubuntu-latest / stable / http3)	33s
bench compile check	50s
test (macos-latest / stable / default)	34s
test (windows-latest / stable / default)	1m 45s
test (ubuntu-latest / stable / rich)	40s
test (ubuntu-latest / stable / default)	33s
test (ubuntu-latest / stable / all-features)	52s
clippy (all-features)	33s
test (ubuntu-latest / beta / default)	34s
test (ubuntu-latest / 1.95 / msrv-rich)	39s
rustfmt	13s
examples build	35m 32s
cargo-deny	31s
mdbook build	5s
fuzz smoke (60s per target)	0s
	50m 23s

Provide feedback