docs: update CHANGELOG for 0.13.0 #1236

Job	Run time
Validate external types appearing in public API	1m 30s
Build rustls-cert-gen as a standalone package	44s
build (macOS-latest, stable)	1m 0s
build-windows	10m 34s
build (ubuntu-latest, stable)	55s
build (ubuntu-latest, beta)	59s
Measure coverage	5m 53s
build (ubuntu-latest, nightly)	53s
Format & clippy	5m 22s
build (ubuntu-latest, stable 7 months ago)	59s
Documentation (stable)	52s
Documentation (nightly)	1m 2s
Check semver compatibility	1m 0s
	31m 43s

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