Fix inconsistent naming of OAuth 2.0 ENABLE setting (#28951)

Renames it to `ENABLED` to be consistent with other settings and
deprecates it.

I believe this change is necessary because other setting groups such as
`attachment`, `cors`, `mailer`, etc. have an `ENABLED` setting, but
`oauth2` is the only one with an `ENABLE` setting, which could cause
confusion for users.

This is no longer a breaking change because `ENABLE` has been set as
deprecated and as an alias to `ENABLED`.

docs/content/administration/config-cheat-sheet.en-us.md

@@ -1107,7 +1107,7 @@ This section only does "set" config, a removed config key from this section won'
 
 ## OAuth2 (`oauth2`)
 
-- `ENABLE`: **true**: Enables OAuth2 provider.
+- `ENABLED`: **true**: Enables OAuth2 provider.
 - `ACCESS_TOKEN_EXPIRATION_TIME`: **3600**: Lifetime of an OAuth2 access token in seconds
 - `REFRESH_TOKEN_EXPIRATION_TIME`: **730**: Lifetime of an OAuth2 refresh token in hours
 - `INVALIDATE_REFRESH_TOKENS`: **false**: Check if refresh token has already been used

docs/content/administration/config-cheat-sheet.zh-cn.md

@@ -1043,7 +1043,7 @@ Gitea 创建以下非唯一队列：
 
 ## OAuth2 (`oauth2`)
 
-- `ENABLE`: **true**：启用OAuth2提供者。
+- `ENABLED`: **true**：启用OAuth2提供者。
 - `ACCESS_TOKEN_EXPIRATION_TIME`：**3600**：OAuth2访问令牌的生命周期，以秒为单位。
 - `REFRESH_TOKEN_EXPIRATION_TIME`：**730**：OAuth2刷新令牌的生命周期，以小时为单位。
 - `INVALIDATE_REFRESH_TOKENS`：**false**：检查刷新令牌是否已被使用。