Merge java/openjdk-*.xml into single java/openjdk.xml

[Copilot]

Consolidates 18 per-version OpenJDK feed files (openjdk-9.xml through openjdk-26.xml) into a single java/openjdk.xml, and renames openjdk-26.xml.template / openjdk-26.watch.py to drop the version suffix.

Structure of merged openjdk.xml

Groups are merged where arch attribute and command set are identical; otherwise kept separate. Results in 11 groups:

Groups	arch	Versions	Notes
1	Linux-x86_64	9–10	servertool, appletviewer, idlj, schemagen, ws* tools
2	Linux-x86_64	11	stripped legacy tools, no jfr
3	Linux-x86_64	12–15	adds jfr
4	(none)	16–18	same commands as group 3; multi-arch impls
5	(none)	19–23	drops rmid/jjs/jaotc/rmic; adds jpackage, jwebserver
6	(none)	24–26	adds jnativescan
7	Windows-x86_64	9–10
8	Windows-x86_64	11
9	Windows-x86_64	12–18	adds jfr
10	Windows-x86_64	19–23
11	Windows-x86_64	24–26	adds jnativescan

All 98 implementations from the source files are preserved.

Reference updates

• jdk.xml: 18 individual <feed> elements replaced with one:

  <feed src="https://apps.0install.net/java/openjdk.xml"/>

• openjdk.xml.template: <feed-for> updated from openjdk-26.xml → openjdk.xml
• openjdk-26.watch.py → openjdk.watch.py (no content change)

Bug fix

Pre-existing copy-paste bug in versions 16–25: implementations inside arch="Windows-x86_64" groups carried a spurious arch="Linux-x86_64" attribute (overriding the group, making them invisible to Windows clients). These incorrect attributes are removed in the merged file.

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