Make ALTER ROLE edgedb refer to admin if edgedb doesn't exist

[msullivan]

This makes mirrors the logic for logins and avoid breaking setup
scripts, such as those in our client test suites and possibly in some
user suites.

Follow-up to #8010.
Makes geldata/gel-python#550 unnecessary.

[aljazerzen]

After this got merged, I'm no longer able to connect to edgedb over postgresql protocol using this command:

$ psql "postgres://edgedb@localhost:5656?sslmode=require"
psql: error: connection to server at "localhost" (::1), port 5656 failed: FATAL:  authentication failed

... but user admin works:

$ psql "postgres://admin@localhost:5656?sslmode=require"
psql (17.0, server 13.9)
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, compression: off, ALPN: none)
Type "help" for help.

edgedb=#

[msullivan]

After this got merged, I'm no longer able to connect to edgedb over postgresql protocol using this command:

$ psql "postgres://edgedb@localhost:5656?sslmode=require"
psql: error: connection to server at "localhost" (::1), port 5656 failed: FATAL:  authentication failed

... but user admin works:

$ psql "postgres://admin@localhost:5656?sslmode=require"
psql (17.0, server 13.9)
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, compression: off, ALPN: none)
Type "help" for help.

edgedb=#

Fixed this in #8057