Update rustls requirement from 0.20 to 0.21 in /gremlin-client

[dependabot[bot]]

Updates the requirements on rustls to permit the latest version.

Release notes

Sourced from rustls's releases.

0.21.7

• Fixes interoperability bug where Rustls servers would not properly echo the session_id in HelloRetryRequest messages as was done for ServerHello messages.
• WebPkiVerifier now stores an Arc<RootCertStore>.
• Documentation for the computational expense required to build client and server configurations has been clarified to emphasize this is cheap with the exception of gathering certificates from a platform trust root store.

What's Changed

• Backport Arc server roots + non-expensive configs to rel-0.21 by @​djc in rustls/rustls#1416
• Backport Echo session_id in HRR by @​cpu, @​ctz in rustls/rustls#1425
• rustls v0.21.7 release prep by @​cpu in rustls/rustls#1425

Full Changelog: https://github.com/rustls/rustls/compare/v/0.21.6...v/0.21.7

Commits

• c9cfe34 Cargo: bump version v0.21.6 -> v0.21.7.
• 41515ea client: detect HRR with incorrect session_id
• 8bf4671 Regression test for echoing session id in HRR
• 0cc2ad5 server: echo client's session_id in HRR
• 0eeac58 Clarify that building configs is not actually expensive
• 9d4ac86 Store an Arc<RootCertStore> in WebPkiVerifier
• 40e4b5d Cargo: bump version 0.21.5 -> 0.21.6
• ea71de2 deps: update to webpki-roots 0.25, fix deprecations.
• 2b97c8d verify: avoid deprecated webpki methods.
• 2ac3cec anchors: deprecate add_server_trust_anchors.
• Additional commits viewable in compare view

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[dependabot[bot]]

Superseded by #199.