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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Content directory changes

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Source	Preview	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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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[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 👀