prepare 8.1.0 release

[LaunchDarklyReleaseBot]

[8.1.0] - 2023-10-17

Changed:

• Build with Go 1.21 and 1.20 in CI
• Bump minimum required Go version to 1.19

Removed:

• Removed obsolete documentation related to disableInternalUsageMetrics config option.

[cwaldren-ld]

Note: the doc change doesn't show up in the diff because it was already merged to public.