Use `DEBUG` to log `invalid authorization header` in `OAuth2TokenExtractor`

[ikhoon]

Motivation:

Invalid authorization is a client error so there isn't much a server maintainer can do. It seems better to use DEBUG to detect problems during development. WARN could be noisy because it may be related to a monitoring system.

Modifications:

• Use DEBUG to log an invalid authorization header.

Result:

An invalid authorization header is now logged at DEBUG by the OAuth2 token extractor.

[github-actions[bot]]

🔍 Build Scan® (commit: def7e03e300aae148aead90b6ef0637ec7c18ebc)

Job name	Status	Build Scan®
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/tpx5vkx4e62na
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/zpoa2exbqgzjo
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/6iq6ncwdwkps6
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/e6y6edp6zye4m
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/giae7iyg22j6u
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/3b6leqf264bno
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/4ycql7ljtbsj6