Add release notes and blog entry for Camel 3.21.0

content/blog/2023/06/RELEASE-3.21.0/index.md

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+---
+title: "RELEASE 3.21.0"
+date: 2023-06-30
+draft: false
+authors: [gzurowski]
+categories: ["Releases"]
+preview: "The Camel community announces the immediate availability of the new Camel 3.21.0 LTS release"
+---
+
+The Camel community announces the immediate availability of Camel 3.21.0, a new LTS release with 295 fixes and improvements.
+
+The artifacts are published and ready for you to download from the Central Maven repository. For more details please take a look at the [release notes](/releases/release-3.21.0/).
+
+Many thanks to all who made this release possible.
+
+On behalf of the Camel PMC,  
+Gregor Zurowski

content/releases/release-3.21.0.md

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+---
+date: 2023-06-30
+eol: 2024-06-30
+draft: false
+type: release-note
+version: 3.21.0
+title: "Release 3.21.0"
+preview: "New LTS release 3.21.0"
+apiBreaking: ""
+knownIssues: ""
+jiraVersionId: 12352568
+category: camel
+kind: lts
+jdk: [11, 17]
+---
+
+This release is the new Camel 3.21.0 LTS release.