Update checking-for-existing-gpg-keys.md

[hemashushu]

Add a workaround for the “No valid identities found in this GPG data” error.

Why:

Users may encounter “No valid identities found in this GPG data” error when importing larger GPG public keys. This is because some users have been using GPG for a long time and the public key contains more signature information. The original documentation does not indicate why this error is encountered and does not provide a workaround.

What's being changed (if available, include any code snippets, screenshots, or gifs):

The following paragraph was added:

If you encounter "No valid identities found in this GPG data" error, it may be because the public key you exported is too large or contains useless information, please add GPG option --export-options export-minimal to export again, e.g.: $ gpg --armor --export-options export-minimal --export 3AA5C34371567BD2

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Source	Review	Production	What Changed
authentication/managing-commit-signature-verification/checking-for-existing-gpg-keys.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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[nguyenalex836]

@hemashushu Thanks so much for opening a PR! I'll get this triaged for review ✨

[hemashushu]

I am providing two public key files for testing:
hippospark.public.zip
one is the original public key, which causes import errors, and the other is the public key after a "gpg clean" operation.
They are both valid public keys and can be verified using the gpg command:

$ gpg --import --import-options show-only hippospark.public.asc 
pub   rsa2048 2013-04-15 [SC] [expires: 2050-03-27]
      AC1F548ADA01B4770A7C5B0E240CA6A5BDAF9022
sub   rsa2048 2013-04-15 [E] [expires: 2050-03-27]
sub   rsa2048 2013-05-14 [S] [expires: 2050-03-27]

[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀

[github-actions]

This is a gentle bump for the docs team that this PR is waiting for technical review.

[subatoi]

Hi @hemashushu, after discussing this internally we'd suggest a minor edit—if you have any specific concerns with this, please let me know, else I'm happy to get this merged. Thank you for your interest in the GitHub docs!

[nguyenalex836]

@hemashushu I went ahead and accepted @subatoi's suggestion, and will get this merged shortly 💛 Thank you so much for contributing to GitHub Docs! ✨

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