docs: Link to Vonage in README

[SMadani]

New customers should sign up for Vonage instead of OpenTok. This is noted in README.md.

[codecov]

Codecov Report

Attention: 103 lines in your changes are missing coverage. Please review.

Comparison is base (242ef4d) 74.41701% compared to head (6c9135f) 75.82938%.
Report is 16 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
src/main/java/com/opentok/Archive.java	95.74468% <100.00000%> (+0.09252%)	⬆️
src/main/java/com/opentok/ArchiveLayout.java	100.00000% <ø> (ø)
src/main/java/com/opentok/ArchiveList.java	100.00000% <ø> (ø)
src/main/java/com/opentok/ArchiveMode.java	100.00000% <100.00000%> (ø)
src/main/java/com/opentok/AudioConnector.java	100.00000% <100.00000%> (ø)
...ain/java/com/opentok/AudioConnectorProperties.java	100.00000% <100.00000%> (ø)
src/main/java/com/opentok/BroadcastLayout.java	100.00000% <ø> (ø)
src/main/java/com/opentok/CreatedSession.java	75.00000% <ø> (+33.33333%)	⬆️
src/main/java/com/opentok/Hls.java	100.00000% <ø> (ø)
src/main/java/com/opentok/MediaMode.java	100.00000% <ø> (ø)
... and 34 more