fix: update watch_ctx.revision to avoid multiple resyncs

[Revolyssup]

Description

Fixes # (issue)
Recently this PR was merged #12015

Although it fixed the original issue, it didn't update revision on the watch context which caused resync to happen again and again. This resync will keep happening until the revision will catch up to the watch_ctx.rev.

2025/03/05 22:52:58 [error] 269#269: *7393 [lua] config_etcd.lua:267: received smaller revision, rev=19, watch_ctx.rev=41. etcd may be restarted. resyncing...., context: ngx.timer
2025/03/05 22:52:58 [error] 269#269: *113 [lua] config_etcd.lua:647: sync_data(): waitdir [/apisix/upstreams] err: restarted, will read the configuration again via readdir, context: ngx.timer
2025/03/05 22:53:42 [error] 267#267: *8621 [lua] config_etcd.lua:267: received smaller revision, rev=20, watch_ctx.rev=41. etcd may be restarted. resyncing...., context: ngx.timer
2025/03/05 22:53:42 [error] 272#272: *36 [lua] config_etcd.lua:647: sync_data(): waitdir [/apisix/protos] err: restarted, will read the configuration again via readdir, context: ngx.timer

This change updates watch_ctx.revision to the last observed revision so that once resync takes place it doesn't happen again and again.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)