Prefer native parser for SSH public key parsing (#23798)

Without this patch, the setting SSH.StartBuiltinServer decides whether
the native (Go) implementation is used rather than calling 'ssh-keygen'.
It's possible for 'using ssh-keygen' and 'using the built-in server' to
be independent.
In fact, the gitea rootless container doesn't ship ssh-keygen and can be
configured to use the host's SSH server - which will cause the public
key parsing mechanism to break.

This commit changes the decision to be based on SSH.KeygenPath instead.
Any existing configurations with a custom KeygenPath set will continue
to function. The new default value of '' selects the native version. The
downside of this approach is that anyone who has relying on plain
'ssh-keygen' to have special properties will now be using the native
version instead.
I assume the exec-variant is only there because /x/crypto/ssh didn't
support ssh-ed25519 until 2016. I don't see any other reason for using
it so it might be an acceptable risk.

Fixes #23363

EDIT: this message was garbled when I tried to get the commit
description back in.. Trying to reconstruct it:

## :warning: BREAKING :warning: Users who don't have SSH.KeygenPath
explicitly set and rely on the ssh-keygen binary need to set
SSH.KeygenPath to 'ssh-keygen' in order to be able to continue using it
for public key parsing.

There was something else but I can't remember at the moment.

EDIT2: It was about `make test` and `make lint`. Can't get them to run.
To reproduce the issue, I installed `golang` in `docker.io/node:16` and
got:
```
...
go: mvdan.cc/xurls/v2@v2.4.0: unknown revision mvdan.cc/xurls/v2.4.0
go: gotest.tools/v3@v3.4.0: unknown revision gotest.tools/v3.4.0
...
go: gotest.tools/v3@v3.0.3: unknown revision gotest.tools/v3.0.3
...
go: error loading module requirements
```

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>

1 files changed, 2 insertions, 2 deletions

diff --git a/custom/conf/app.example.ini b/custom/conf/app.example.ini
index 0543c85d50..88297fe0d9 100644
--- a/custom/conf/app.example.ini
+++ b/custom/conf/app.example.ini
@@ -186,8 +186,8 @@ RUN_MODE = ; prod
 ;; default is the system temporary directory.
 ;SSH_KEY_TEST_PATH =
 ;;
-;; Path to ssh-keygen, default is 'ssh-keygen' which means the shell is responsible for finding out which one to call.
-;SSH_KEYGEN_PATH = ssh-keygen
+;; Use `ssh-keygen` to parse public SSH keys. The value is passed to the shell. By default, Gitea does the parsing itself.
+;SSH_KEYGEN_PATH =
 ;;
 ;; Enable SSH Authorized Key Backup when rewriting all keys, default is true
 ;SSH_AUTHORIZED_KEYS_BACKUP = true