Fixes #5558 : Skip JdkObsolete Charset API suggestions when source level lacks them

[mirkoalicastro]

The JdkObsolete checker recommends Charset-accepting API alternatives (for example URLEncoder.encode(String, Charset)) that were only added in JDK 10. Users compiling with --release 9 or lower cannot use the suggested alternatives.

Changes:

• Adds a jdkVersion field to ObsoleteApi (analogous to the existing androidMinSdkVersion) and checks the compilation source level before reporting these APIs as obsolete.
• Sets jdkVersion=10 for all entries whose Charset alternatives were introduced in JDK 10.
• Entries whose Charset alternatives have existed since JDK 1.6 (String, InputStreamReader, ..) are unaffected.

Fixes #5558

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