RFR: added jdk21 to test matrix

[Jiří Vaněk]

This is adding jdk21 to jdk test matrix and removing at8 branch, which is no longer used
I had not added at7 branch.
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### 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>

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Commit messages:
 - added jdk21 to test matrix

Changes: https://git.openjdk.org/asmtools/pull/72/files
 Webrev: https://webrevs.openjdk.org/?repo=asmtools&pr=72&range=00
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/asmtools/pull/72.diff
  Fetch: git fetch https://git.openjdk.org/asmtools.git pull/72/head:pull/72

PR: https://git.openjdk.org/asmtools/pull/72