docs: fixes for release notes for 1.20 (envoyproxy#18403)

cleaning up release notes

Risk Level: n/a
Testing: n/a
Docs Changes: yes
Release Notes: no
Signed-off-by: Alyssa Wilk <[email protected]>

v1.18.4 release

Signed-off-by: Lizan Zhou <[email protected]>

v1.17.4 release

Signed-off-by: Lizan Zhou <[email protected]>

v1.16.5 release

Signed-off-by: Lizan Zhou <[email protected]>

release: v1.19.0 (envoyproxy#17320)

Signed-off-by: Matt Klein <[email protected]>

v1.18.3 release

Signed-off-by: Yan Avlasov <[email protected]>

v1.17.3 release

Signed-off-by: Yan Avlasov <[email protected]>

v1.16.4 release

Signed-off-by: Yan Avlasov <[email protected]>

v1.15.5 release

Signed-off-by: Yan Avlasov <[email protected]>