fix(deps): update dependency com.google.cloud:google-cloud-core to v2.21.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.cloud:google-cloud-core	2.20.0 -> 2.21.0

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Release Notes

googleapis/sdk-platform-java (com.google.cloud:google-cloud-core)

v2.21.0

Features

• Dynamic Routing Headers for HttpJson (#​1667) (003b993)
• Implement awaitTermination() for MangedHttpJsonChannel (#​1677) (dea8426)

Bug Fixes

• [gapic-generator-java] align writer behavior for nested types (#​1709) (a21ffe8)
• [gapic-generator-java] update year to 2023 in generated license headers (#​1720) (ef4a7ff)
• Update the return type of setWaitTimeout for convience (#​1751) (f1927a5)

Dependencies

• Update dependency com.google.auth:google-auth-library-credentials and google-auth-library-oauth2-http to v1.17.0 (#​1756) (f583258)
• Update protobuf dependencies to v3.23.2 (#​1755) (55ecbc3)

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