db_options: Impl Default for EncodingType manually

This avoids having to bump the MSRV upstream.

src/db_options.rs

@@ -3584,10 +3584,9 @@ pub enum ChecksumType {
 
 /// Used by [`PlainTableFactoryOptions`]
 #[repr(u8)]
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum EncodingType {
     /// Always write full keys without any special encoding.
-    #[default]
     Plain = 0,
     /// Find opportunity to write the same prefix once for multiple rows.
     /// In some cases, when a key follows a previous key with the same prefix,
@@ -3603,6 +3602,12 @@ pub enum EncodingType {
     Prefix = 1,
 }
 
+impl Default for EncodingType {
+    fn default() -> Self {
+        Self::Plain
+    }
+}
+
 /// Used with DBOptions::set_plain_table_factory.
 /// See official [wiki](https://github.com/facebook/rocksdb/wiki/PlainTable-Format) for more
 /// information.