RFR: added jdk21 to test matrix

[Jiří Vaněk]

On Thu, 18 Jan 2024 11:27:54 GMT, Jiří Vaněk <jvanek at openjdk.org> wrote:

> This is adding jdk21 to jdk test matrix and removing at8 branch, which is no longer used
> I had not added at7 branch.
> ---------
> ### Progress
> - [x] Change must not contain extraneous whitespace
> 
> ### Error
>  ⚠️ The pull request body must not be empty.
> 
> ### Reviewing
> <details><summary>Using <code>git</code></summary>
> 
> Checkout this PR locally: \
> `$ git fetch https://git.openjdk.org/asmtools.git pull/72/head:pull/72` \
> `$ git checkout pull/72`
> 
> Update a local copy of the PR: \
> `$ git checkout pull/72` \
> `$ git pull https://git.openjdk.org/asmtools.git pull/72/head`
> 
> </details>
> <details><summary>Using Skara CLI tools</summary>
> 
> Checkout this PR locally: \
> `$ git pr checkout 72`
> 
> View PR using the GUI difftool: \
> `$ git pr show -t 72`
> 
> </details>
> <details><summary>Using diff file</summary>
> 
> Download this PR as a diff file: \
> <a href="https://git.openjdk.org/asmtools/pull/72.diff">https://git.openjdk.org/asmtools/pull/72.diff</a>
> 
> </details>

@lkuskov afik this PR is ok. IDK why the GH do not run here. In my fork, it run correctly under  jdk21

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PR Comment: https://git.openjdk.org/asmtools/pull/72#issuecomment-1948373134