Add mavenCentral as plugin repository

[trivikr]

Issue #, if available:
Internal JS-3822

Description of changes:

CI build are failing with:

A problem occurred configuring project ':smithy-typescript-codegen-test'.
> Could not resolve all artifacts for configuration ':smithy-typescript-codegen-test:classpath'.
   > Could not resolve software.amazon.smithy:smithy-cli:[1.26.0,1.27.0[.
     Required by:
         project :smithy-typescript-codegen-test
      > Failed to list versions for software.amazon.smithy:smithy-cli.
         > Unable to load Maven meta-data from https://plugins.gradle.org/m2/software/amazon/smithy/smithy-cli/maven-metadata.xml.
            > Could not get resource 'https://plugins.gradle.org/m2/software/amazon/smithy/smithy-cli/maven-metadata.xml'.
               > Could not GET 'https://jcenter.bintray.com/software/amazon/smithy/smithy-cli/maven-metadata.xml'.
                  > Read timed out
   > Could not resolve software.amazon.smithy:smithy-gradle-plugin:0.6.0.
     Required by:
         project :smithy-typescript-codegen-test > software.amazon.smithy:software.amazon.smithy.gradle.plugin:0.6.0
      > Skipped due to earlier error

This is happening due to jCenter being shutdown.

The mavenCentral was added as plugin repository in other places in smithy-typescript, and in smithy project in smithy-lang/smithy#1189. This PR adds mavenCentral for smithy-typescript.

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