Expanded minimum RSA Keylength to 3072 (#26604)

Expanded minimum RSA Keylength to 3072 (#26604)

German Federal Office for Information Security requests in its technical
guideline BSI TR-02102-1 RSA Keylength not shorter than 3000bits
starting 2024, in the year 2023 3000bits as a recommendation. Gitea
should request longer RSA Keys by default in favor of security and drop
old clients which do not support longer keys.

https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/TechnischeRichtlinien/TR02102/BSI-TR-02102.pdf?__blob=publicationFile&v=9
- Page 19, Table 1.2

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>